Even as a former citizen of the CCCP, i feel a bit of shame in not knowing the answer to this question. I never understood why they weren't, same goes for other countries. Any help guys?
Since Mongolia was effectively under the control of the USSR, the other Warsaw Pact countries didn't want to admit Mongolia (even though Mongolia tried) and give the USSR even more power.
Admitting Mongolia would needlessly antagonize China.
After 1966, the Soviet Union had troops in Mongolia at all times, so aside from the politics of the issue, there was no need to join the Pact (for the protective aspect, at least).