They didn't forget, they tried to but were refused. And, the refusal wasn't because of restraint from the USSR (as potentially implied when this topic came up in another thread, but was off-topic in that thread, and perhaps offtopic in this thread, but I've been itching to respond to that statement, but not wanting to start a whole new thread about it).
Mongolia was more useful to them as a compliant puppet (like current day Belarus is) than to increase the chances of conflict with China. Current day Mongolia not the puppet to Russia like it was to the USSR.