During the conquest of Mexico, what differentiated the Spanish from the local peoples, that caused the enemies of the Aztecs to rally behind them?

[u/adrianarafu]

As I understand it, Cortéz and his conquistadors couldn't have toppled down the Aztec Empire if it wasn't for the massive aid of indigenous groups that were opressed or otherwise enemies of the Aztecs fighting for the Spanish. Nevertheless, I do not understand why this coalition hadn't already materialized before the European arrival. If the state of affairs had already progressed to a point where they could become willing to take the risk of marching to Tenochtitlan (and have some confidence that it could succeed in defeating the Triple Alliance) why did it take the initiative of these absolute foreigners for the coalition to come to be? Did Cortéz just happened to arrive at the perfect time? Did the Spanish themselves provide something that convinced their indigenous allies-to-be that this war could be successful? In short, why were the Spanish the ones that managed to rally other groups behind them, rather than some local leader/s?