Umm, no, you're confusing it with StuH and brummbar.
III E didn't had any howitzer, only low velocity gun. It was meant to be close infantry support and destroy soft targets and fortifications. Basically things that soviets don't have.
Also(yes, I'm going to say it, because why not, its used as argument in soviet case) you pay no tech costs to get this cheap armor.
Dude..
IS-2, IS-152, T34/85, Tiger & Elefant doesn't require teccing so the armor has to be cheap.
Nice argument MVgame