Some thoughts…
While the Soviet Union and Communist China were/are undeniably authoritarian, repressive regimes, it's hard to argue that any symbol has become as indelibly intertwined with racism, hatred, and genocide as the swastika. There simply is no comparison in modern history. Certainly Nazi Germany had no monopoly on racism and genocide, but there is no other Western precedent for a nation that initiated a program of systemic, state-sanctioned racism and genocide on the scale of the Nazis, from the Nuremberg Laws of 1935 onwards.
I think there's also a cultural component to the issue as well. Historically, Russia was never viewed by Western Europeans as a part of Europe. And China, obviously, was on the other side of the world. Germany was one of the traditional great powers of Western Europe, a sibling nation that gave itself over to state-sponsored racism, declared itself the "master race," and then embarked on a campaign of conquest that resulted in other Western European states being subjected to its programs of terror and genocide. Framed in that light, it's not difficult to see why Western European nations felt compelled to outlaw Nazi symbols in the war's aftermath. As for Eastern European states, they may have ended up under the Soviet bootheel, but they initially welcomed the Soviets as liberators. And the Soviets themselves played a significant role in abolishing the symbols of their hated adversary in their vassal states.