Diseases were affecting both Aztecs and their enemies. Also, while USA did not had to fight "peaceful" and well developed farmer societies in Northern American plains, organized Comanche raiders that came from north and subjugated whatever was spared by plagues were a headache for colonists for couple of centuries.

What most of the comments along with article writer are missing - it was not just technology, that Spanish used, but organization structure and command of professional military forces - that is one of the reason why 1000s of peasants were incapable to fight against couple of men in armor, as they lacked coordination - that would be the same as pitting street kids against a swat team, even if they were given the same weapons.

The same applies to Aztecs that had different POV and mindset against battle-hardened professional European soldiers - when you read how Aztec demise unfolded, in some of it you have to question what they were thinking, but that is mostly because of our modern bias, that has not much knowledge of their beliefs on how they viewed world around them.

Also, not more important fact - now, just like then, not all people were interested to be involved in war, and a peace agreement for them was even better than being part of alliance(and the choices here are to die for foreign Aztecs as one of them) and join fight. And those who already hated Aztecs to the guts, also had plenty of motivation to see all of that empire burn.

PS. People who are mentioning bunch of foreigners are clueless about European history - whole European FEUDALISM political system was born out of bunch of foreign raiders, that we know today as vikings and their descendants, that carved up Europe and limited their in-fighting with rules and brought unified force against those that challenged them. Europeans had pretty much the same scenario before repeated on native Americans and many times.