This is sort of a just-so story that isn't completely true. Stalin pushed "socialist classicism" [1] as an aesthetic, and it wasn't until his successors took over that modernist architectural ideas imported from the West became a hallmark of actually-existing socialism.
There was a whole decade of Soviet design before Stalin, which was often radically modernist even by the standards of its day, and that is what the OP was referring to. Stalin was notoriously conservative in his artistic tastes.
I said early Soviet for a reason.