As noted the Armenians had support from the Russians, as well as what I would argue was a more determined resolve to take control over Nagorno-Karabakh. The Armenians also had the support of their diaspora, of which the American-based Armenian community was able to lobby Congress to forbid any aid to go to Azerbaijan in the form of Section 907 of the Freedom Support Act, which had a serious effect (this lasted until September 11, when the President was given the ability to waive it for countries supporting the War on Terror). And simply having a large number of weapons and soldiers does not guarantee success, as has been proven repeatedly throughout history. Azerbaijan was quite disorganised in the post-Soviet breakup, and the government was not able to consolidate itself until Heydar Aliev took control in 1993; in contrast Armenia had a united front and was able to use this to exploit the Azeri situation to their advantage.