The Red Army in fact went through several helmets during the war so it's a myth that they didn't equip their troops with helmets. Indeed a google search of "Red Army WW2 helmets" will return dozens of results.

That said I have noticed soft caps in many Red Army photos, so I understand your question. And the simple answer as to why is the fact that many of these photos were posed shots and not actual combat shots - often of officers or commissars.