There was no real animation industry in the USSR before the war. Soyuzmultfilm started up in 1936, but then the war came around and the animators had to evacuate behind the Urals. There was also no color film in the USSR before the war either - they took it from the Germans after the Allied occupation began. (Harsh, you may say, but when you go on a genocidal invasion some things are fair game.)
There are two reasons why these films were so good and memorable. For one, the animators had relatively free hands (there were still things you couldn't do, as there is here at Western companies) because their films brought in awards and prestige to the government. And for the other, well, everyone has to be entertained, and the officials in charge knew that. There were many fine comedies made in the Soviet Union that everyone in modern Russia still knows and loves, but you never hear about them in the West. Same with all the other Eastern Bloc countries.