system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why do modern battleships only have one small canon compare to old battleships that had at least 9 huge ones?
gpt
Air power:

In 1950 it became obvious that guided missiles and bombs could hit a ship easily. At that point the aircraft bombs and missiles were available in massive sizes compared to 10-20 years before. For massive, to be clear, it means bus size. Those bombs/missiles had the capability to make a hole in a 30 inch thick armor plate. The only ship big enough to carry such armor would have been a super battleship 1km long and 10 time the cost of ww2 battleships. Even if you armor against the missile, your armor bent can make a bigger missile quicker than you can build the new bigger battleship.

Logical solution was to give up armor completely, make “cheaper” smaller ships and use the space and displacement to carry the biggest possible punch. The biggest punch you can get, is a cruise missile, there is no gun that can punch that hard. 

Still, an enemy can hypothetically jump you at close range using an island as a cover, or whatever sort of ambush you could plan. So you still need something to insta-kill at close range (naval point blank range is 10km nowadays, for scale). But as we said, there are no armored ships around, whatever ship ambushes you, will be unprotected as you are. So you just need to mission kill your ambushed before he fires. To mission kill an unprotected ship, you can just use a “small” fast firing gun. 

And there we are, all missile cruisers carry a single (or double) turret with a fast firing gun, that is 5-6 inch in caliber, more than enough to cripple a 300m long cruiser in 1 minute, by shooting few dozens 10kg shells.

For comparison, this cannon is super tiny compared to ww2 battleship guns, which shells weigh 1-2 thousand kg, and were armor piercing up to 16-20 inch of armor. And because the fight were at 20-30km range, they needed many guns to spray the area and maybe get a kill at that range. That range is non existent now, because a missile can go 200km or more, again, the only gunfight you can get is a strange ambush that never happens, the “tiny” gun is there just in case.

Minor editing for clarity.

Thanks for the award!!!