Because the shoe is attached to the hoof when it is in its most contracted state  e.g. when lifted off the ground while the farrier applies it. This metal rim does not allow the hoof to expand. It works like a cast.  The nails in addition to the damaging effects of constant contraction, damage the laminar horn and therefore the integrity of the hoof, opening it to bacterial invasion (old nail holes) and temperature changes (as metal conducts heat or cold to the inside of the hoof).
Traction and footing is altered as well, as there is no flexibility in the hoof. It may either be too slippery (on concrete, ice, wet grass etc.) or will provide too much traction in deeper ground for example - which can lead amongst other things, to ligament, tendon and muscle strains.
The horse simply does not feel the ground it's walking on.
Because of a number of superficial advantages (!) of shoeing, we tend to use the horse beyond its biological limitations, not noticing that an organism can cope with unnatural demands only for a limited period of time until eventually "something has to give": The horse may develop what is only too common amongst domestic /performance horses all over the world: Navicular syndrome, ring bone, side bone, splints, white line disease, cracks, tendon problems, muscular problems, laminitis, arthritis, contraction, club foot, coon foot , as well as most "conformation" problems etc. until they finally becomes "incurably" unsound despite desperate efforts of veterinarians and farriers. 
Note: The effect of incorrect trimming can be similar to shoeing. Why? Because....

Hoofmechanism is only possible when the hoof is trimmed in such a way that its shape and dimensions facilitate this function.

If for example excessive high heels and/or bars do not allow the hoofcapsule to widen and draw flat on weightbearing, then the effect would be similar to the effect of the metal brace of a shoe.

So what can the Strasser method of Holistic Hoofcare do for you -  and more importantly for your horse?
It can restore a functional healthy and natural hoof though trimming techniques that optimise hoofmechanism and establish a ground parallel coffin bone (for optimal balanced weight distribution).
When these conditions have been achieved, lameness issues can be prevented, as well as rehabilitated, because healing can take place.
Depending on the damage within the hoof, rehabilitation may take months or even years, but the results will always be achieved through true healing and not the suppression of symptoms.
What took a long time to develop will not heal over night, that's why owner involvement and dedication to provide favourable conditions (a natural lifestyle for example) is so critical.

Before we look a little closer at Dr. Strasser's method of Holistic Hoofcare (which includes lameness Prevention and Rehabilitation as well as Hoof Orthopaedics), we might  define what the qualities of a healthy hoof are:

If your horse' s hooves are shod, hoofmechanism can not take place and most of the above functions and qualities are impaired or altered.
What is hoofmechanism?
Hoofmechanism is the reversible deformation of the hoofcapsule: It expands and draws flatter when weightbearing and contracts when unloaded.
This mechanical alteration of shape has a suction/pumping effect during which the highly vascular corium fills with blood (when the hoof is weightbearing and at its widest) and squeezes venous blood back up and out into the coronary venous plexus when lifted). Every step the horse makes, assists the heart to circulate blood through the body. So does muscle activity.
If the horse is kept from moving (stabled) and is shod as well, the heart has to work over-time. Cardio-vascular problems can be the result.

The lack of circulation through the hoof means lack of metabolism, e.g. in horn production. That is why long term shod horses can have inferior horn quality and shoeing becomes a "necessary evil"  because the horn quality is that inferior that shoes are needed .. until they no longer stay on.
(Horses are being put down because there is nothing left to drive in a nail!!!!!)

What is also a relevant fact is that nerves can not transmit stimuli well, if they can not be re-charged to "fire"  due to an insufficient availability of ATP (an energy molecule that re-establishes the nerve's resting potential)  which is the case when circulation is reduced. 
That is why in some cases shoeing (especially orthopaedic shoeing) may give the impression of a "cure".  What it is however, is the effect of reduced circulationNo circulation  no feel. So how can shoeing have such and effect on hoofmechanism and all the functions of the hoof?

No Hoof, No Horse

Horses as we all know them today have evolved over a period of about 55 million years: They have developed from a small five-toed forest dweller to a highly efficient steppe dweller with acute senses, well developed social behaviour and an athletic body with a powerful "engine" to outrun predators.
Their digestive system is efficient to draw nutrients out of large amounts of forage (small stomach, large intestine) , which means they have to continuously graze. It is a physiological as well as psychological need.
Their entire physique as well as their psychological make-up is meant to function best in a herd situation, on pasture and with the freedom to move around at liberty 24/7 and because of the necessity to find food and water.

The biological needs of equines have not changed in millions of years

Horses did not need to wear clothing, eat high energy / processed and concentrated feed, hide in caves or wear footwear on their hooves to survive the Ice Ages and all the different conditions in climate or ground they encountered.
In fact, they did pretty well, until we discovered them as "useful" and domesticated them.

Perhaps now it is time to re-discover what horses are and what makes them "tick".
If we would consider their biological needs more, we may be able to enjoy them more and longer without (unintentionally) causing detriment to their longevity. Unfortunately, this in many cases is exactly what we are doing.

Hoofcare in particular as we understand and practise it, has to take into account the entire organism  "horse" and all its biological needs.
Overlooking the importance of healthy hooves, and their connection to the metabolic and cardio-vascular health of the horse will always lead to unsatisfactory attempts to prolong useability "for just a little longer", making genuine attempts of rehabilitation more difficult and time consuming.

Dr. Hiltrud Strasser, a German Veterinarian, has spent more than 20 years of research and study, documenting "modern" hoof pathologies and their origin and holistic ways of rehabilitation.
Her findings are taking the world (and many desperate horse-owners) by storm.. which means in some minds her holistic approach to hoofcare is simply too "revolutionary"! Her findings seem to challenge most traditional, conventional beliefs.                                                                                                         

1. Good hoofmechanism
2. A ground parallel coffin bone
3. Full range of functions such as
   a) Protection of inner structures
   b) Good traction and secure footing on all terrain
   c) Shock absorption
   d) Circulatory pump of corium
   e) Protein excretion (horn growth)
4. Free of pain/ no lameness
5. A symmetrical, balanced appearance with appropriate     dimensions and size.
6. Growth rate of horn balances with wear
7. Elastic horn due to regular exposure to water

It is just a fact that every horse is born with all the footwear it ever needs.
Did you know that the majority of domestic horses have deformed feet?
What about your horse?
Nonsense, you say, neither your farrier nor your vet have ever made such a statement. After all, your horse is not lame, has the best of care and gets shod/trimmed regularly!
Unfortunately, in most modern, conventional veterinary and farrier text- and reference books, hooves are depicted and described as healthy and anatomically correct when they are in fact pathologically deformed.
A healthy hoof shape, the functions of the hooves and their importance to the entire organism is insufficiently represented or completely misunderstood by professional and laymen alike.

We have learned to accept as healthy and "normal" what in reality is pathological,
dysfunctional and deformed!

Because of these fundamental misunderstandings we often bring about  (unintentionally, of course!)  early lameness and untimely destruction of horses.

Looking back, our traditional/conventional hoofcare as still practised by our farriers today, originates in the Middle Ages, when the dawn of the metal shoe coincided with the first documentation of the same lameness problems which our modern horses are presenting with today.
It was also in the Middle Ages where the stabling of horses became common practise.
For some reason, it now is commonly accepted to be "good practise" and deemed to be "good husbandry" if horses are kept stabled, shod, rugged and kept in solitude.
By removing horses (or any creature) from their natural habitat/environment we will cause physical and emotional distress.or worse.
Horse have evolved to feel safe in a herd, roaming open plains and pastures. Boarding them in small confined spaces like stables and yards where they have to stand in an unnatural posture for most of the day and night  and on some form of bedding in with their own excrements, we changed their quality of life drastically to the worse
We may not quite understand their need for freedom of movement, as most of us yet have to learn that what may be good and convenient to us, may in fact be detrimental to another creature's health and wellbeing...Why? Because horses are biologically, anatomically, physiological as well as psychological very different to us!

No Hoof - No Horse - No Kidding (by Carola Adolf NEP/fSHP)