Why did the USSR break apart at ethnic lines? Because it organized itself around national self-determination ensuring that each ethnic nation got it's own Republic. The entire structure of the USSR was built around ethnic lines to begin with because they believed it was critical that every ethnicity be self-determining. They even had a constitutional right for each state to secede legally because they felt that if you didn't have the right to secede then you didn't actually have the right to self-determination.

Also, what do you mean by "inherited the Tsardom". You realized they killed the aristocracy and dismantled the entire Tsarist structure right? Or do you think that Vietnam is actually a French colony still, too?