alt Hacker NewsThere's no mystery here. The Aztecs ruled brutally over dozens of other indigenous groups, they practised slavery and human sacrifice. When the Spanish showed impressive military equipment and the implication (when reinforcements arrived, albeit in small numbers) that they represented a powerful external military force, ancestral opponents who'd been subjugated by the Aztec empire were quick to throw in their lot.
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Spanish granted citizenship and protection from the crown to everybody playing under their rules. Spanish crown and Christianity promises should look shocking and also as 'best deal in my life' compared to the previous situation.
And as most military based societies, Aztecs had also a big problem with management of ecology and natural resources, caused with their own development, big cities with a less than optimum management of fecal waters, and never ending wars.
Deforestation came with dry spells and agriculture suffered while they were trying to negotiate a solution with the stars. All the social structure was supported by a lot of drugs abuse, probably.
It wasn't just the captive slaves and human sacrifice either (though that was a big deal), the Aztecs imposed crushing economic tribute burdens on subjugated city-states, and when payments fell short, they'd go in and raid. Tribute wasn't just gold and tradeable goods, it was also food, and the Aztecs didn't care if they drove a place into famine.
There were also rival indigenous states (Tlaxcala, etc.) that were never conquered by the Aztecs but had been trapped in continuous low-intensity war with them for years.
Though it is worth saying the Aztecs weren't universally despised. Some regional dynasties were allies. Some of those were also unpopular.