system
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human
[Star Wars] As a whole, why were the Jedi and Sith more powerful in the old republic area than the republic era?

I know the republic era had it's fair share of powerful Jedi and Sith but on the whole as an order why were they more powerful in the old republic? 

Just a few examples so you get what I'm talking about.

Satele Shan- Can stop a lightsaber with her bare hands as well as shoot force projectiles so powerful she can destroy a small cliff.

Dread Masters- Can render an entire fleet of starships immoble by causing such fear that those targeted either go mad or commit suicide.

Sith emperor- sustained himself for hundreds of years, can feed off the force energy of billions making him stronger all the while sustaining revan and probing his mind.

Darth Nihilus- literally feeds on the force

Countless Sith sorcerers who can literally create terrifying storms of force lightening, Jedi's who can sever someone's connection to the force.

So my question is what the hell happened between the old republic and the republic era before the rise of the empire where both Jedi and Sith lost their prowess over the force?
gpt
Rule of Two, mostly.

The rule of Two was both the best and worst thing that ever happened to the Sith. It accelerated their development within the Force (every Master is probably going to have a wider general knowledge than his Master, as well as his own specialties) and ensured that only the best candidates were selected, in terms of Force potential and personality. It also cut them off from the ruthless infighting, murder and mayhem that comes when you take a large handful of Sith, a pinch of power, a planet and shake well.

It forced them to stop their military opposition of the Jedi and the Republic and start worming their way inside. Seed dark side cults instead of marching on the Jedi temple. Etc. It also caused an awful lot to be lost, since the Master never really revealed everything he had to teach. While the main body of the knowledge passed on was larger, his greatest secrets stayed hidden. Something that might be taught when you were in the Jedi Order for, say, fifty-sixty years, and had been taught for the better part of a millenia, might take two millenia to enter the body of 'general knowledge' for the two Sith. The galaxy is also a bit more peaceful - while still boiling with conflict, there's considerably less outright war and more skirmishing. No more great crusades to forge new Sith revelations, new dark paths to follow.

Now. Take the Jedi Order: The Sith Empire is gone. Vanished into the dark, never to return. The great Vitiate dead, the supposed immortal rotting away into oblivion. Clearly it's time to get shitfaced and party, right? So they do, in a sensible manner as you'd expect from the Jedi - two drinks and an early bed time, with a few glasses of water beforehand and a handful of multivitamins. Then they settle down. They're peacekeepers now! Just what they always wanted. So they get complacent. Aside from the odd flareup, they lose the massive conflicts that kept them shaken up. There's nothing out there, really - border skirmishes, delicate negotiations, hostage crises are nothing compared to an all out war. So they sink into complacency over thousands of years. Noone pushes, noone pulls. Knowledge is lost, or hidden and forgotten. Holocrons roll under the couch. Or stay locked away, because 'there's no need' to learn how to eviscerate fifty soldiers in a blink in the current universe.

The Force Adepts are just as potent as they ever were - Palpatine was casting a pall of darkness over the entire galaxy BEFORE his Ascension - but they've got less to work with and less impetus to grow. Yoda and Sidious were exceptional not just for their strength, but for their willingness to move beyond the accepted body of knowledge. Sidious, for example, could tear the universe a new one and create wormholes as well as summoning force storms. But the rest? They rely on the Force to get what they want done, and rarely grow further - it takes great drive to overcome the inertia. Kreia was right, in a way.