It's an aesthetic choice.Anyone know the answer?
Realistically, most PCs should be kitted out in full plate, because it's just better than other types of armor. The only reasons to not wear plate are a) it doesn't exist in your world or b) you can't afford it*. Neither of these generally applies to D&D adventurers.
But many players want a different visual of their character; the lightly armored duelist, the wizard in flowing robes. So D&D adds assorted balancing factors to make such PCs viable. Limiting Dex bonuses for medium/heavy armor is one such factor. The equally-unrealistic proficiency requirement is another.
*Actually there is a third reason, which is that when you're facing a huge foe like a giant or a dragon, armor is nothing but dead weight. A giant's fist or a dragon's bite will crumple up your fancy plate armor like tinfoil; your only hope is to evade the blow, so you want to be as lightly equipped as possible. But this requires a whole different combat model for D&D, and it also means you lose the visual of the knight in shining armor squaring off against a dragon.