Because it's more effective.
Can kill any tank, but has more ammo and higher ROF, and it's much better armored.
This. This, right here, is why the 37mm option for the Stuka should not, not, NOT be perked if it's made available. Because it is balanced by the fact that it's on A BLOODY STUKA. Slow, vulnerable, with LOL mm's of armor.
On the other hand I don't think the IL-2 in the game is as good a gun platform as the Ju-87, and the Ju-87 is rather more stable and controllable in diving attacks as well (which is the angle one should be using against tanks, if you want to hit the thin top/deck armor.)