It was designed by a WW2 veteran tank operator (presumably a T-34/85), and he had in mind a weapon which would be devastating to unarmoured and lightly armoured enemies at close range when you were in a pinch. It wasn't really designed to be pretty or have long range utilization but be reliable and deadly when you needed it, and it fulfilled that role marvelously.
Mass production reduced the price, so it became the principle weapon of armies and militias all over the world.
"Blah blah blah blah more soviet propaganda on how we made this weapon blah blah blah blah but let's not mention WE COPIED IT FROM THE HATED GERMAN MP43 so instead say some patriotic soldier of the great soviet proletariat came up with the idea whilst eating cabbage and drinking vodka." Not a bad tale easily believable but it was STILL STOLEN FROM THE DESIGN OF THE MP43.