What did the soviet union do wrong to not work out in the end?

So I know a bit about communism, (not even remotely close to this sub though) and think that in its core the idea is good but due to human nature nearly impossible to accomplish in its pure form. The soviet union would be a good example, its economy collapsed after a prolonged arms race with Nato but the US economy didn’t. So what was in your opinion the determining factor of its collapse?This is just my opinion, im not stating this as facts. via /r/communism https://ift.tt/38AgKpd