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.
From thefreedictionary.com:
Normally a cannon with a tube length of 20 to 30 calibers; however, the tube length can exceed 30 calibers and still be considered a howitzer when the high angle fire zoning solution permits range overlap between charges. See also gun; mortar.
The StuGIIIE has low velocity gun of 24 calibers long, firing a shell with comparatively high explosive content compared to the higher velocity variants of the 75mm guns. You could charitably call it a gun-howitzer.
The Soviets also have plenty of soft targets- they're called 'infantry'.