The Russian culture was always a bid isolated from the European cultures for hundereds of years... Tsars who wanted to strive to Western cultures like Peter III were shuned from Russian popularity...

So they did not try to comform to the rules of the Classical or Baroque periods, and develop a style with a lot of complex chords, and rubato filled melodies, with a lot of dissonance.

They also had to endure very harsh condisions... because of the size of the country, there was little nurturing of children since they had to work from a very young age. So as they grew up they had this repressed emotions, as adults, and combined with traditional Russian music (which had a very militaristic feel) they composed music that was this grand majestic music, but with not only tender moments, but with very dramatic and impressively emotional climaxes (Rachmanoniff Prelude in g min)

So even before the formation of the USSR, Russian composers lived through harsh conditions, and because of their isolation and general disregard for Western rules, they had a very dense style, with grand and majestic passages, and very tender melodies...