James Flack -To keep the size, weight and cost down. By sacrificing capacity, they could be smaller. Smaller means lighter and cheaper.
T72 vs M1 size comparison.
Challenger ( UK) vs T62.
<<Insert own joke about obesity in the west>>
As the question mentions elevation and depression… how do you improve those? Make the turret taller, so the breech of the cannon has more room to move. So you need more armour, and weight, for the now larger turret.
Keep under a certain weight, and you can get away with a smaller and lighter engine, gearbox etc