Another reason the Aztecs fell was because of this guy, Montezuma II. Instead of killing the Spaniards as soon as possible, he welcomed them into to the city.
There was a reason for his open-arms policy with the Spanish. There was a Aztec prophecy that a white skinned and bearded god would some day return, take control of the Aztec Empire, and establish world peace.
Do you see why Montezuma was friendly with the Spanish? It would have been a very bad thing if he had ordered the death of Cortez only to find out he was the god from the prophecy. In a religion that practiced human sacrifice, insulting a god was not your best survival plan.
The Spanish were fighting with guns, cannons, trained dogs, light and very sharp swords, and trained horses. The Aztecs were fighting with these:
Heavy, wooden, and outdated, these seemingly fearsome weapons were no match for Spanish weaponry.
Along with more advanced weapons, the Spanish enjoyed greater protection with steel plated armor, while the Aztecs had to do with the skins of wild beasts and their feather plumes.
Not to mention the Spanish would have been taller than the Aztecs.
Montezuma and his people were hated by the surrounding tribes and cities. By expanding their empire with blunt force, the Aztecs were left friendless. The nation-states conquered by the Aztecs were quick to join with the Spanish to overthrow their hated Emperor.
The Spanish were immune carriers of the disease, the Aztecs were not. The disease racked havoc in the Aztec empire, ultimately resulting in the death of thousands.
In the end
Cortez was named governor of the newly conquered territory, and the Aztec empire continued to crumble until it was nothing but the stuff of legend.
Cortez's reign over Mexico was a troubled one. After returning from putting down a rebellion in Honduras, Cortez found his governorship had been stolen. He retired to Spain in 1540. But in the end he did get his face on money.