The Aztecs had a much smaller population, and the majority of those 6 million "inhabitants" sided with the Spanish because they were at permanent war with the Aztecs, who waged wars almost every day to capture victims for human sacrifices.
That's how China got overrun by steppe nomads twice, for example. The unusual thing about the Spanish conquistadores is mostly how little of an initial force they needed to set off an avalanche. Even Genghis Khan had to slowly work to unify the Mongol tribes before he could switch to rapid expansion.
During the wars of independence, the Indigenous people actually fought against independence because they remembered how things had been before and how much better things had become under Spanish rule.