It's in a very high and isolated area. Before roads and trains, it was really difficult to reach.

I think this one happened rapidly because of conquistadors and plague. A rapid empire collapse making the elite location untenable and unaffordable.

I recently read 1491 and it was excellent. It describes how these immense civilizations were in the New World, and how rapidly they collapsed as hogs infected with diseases accidentally escaped the explorer's camps and killed of 90% of the populations.

It talks about De Soto seeing the banks of rivers filled with dozens of cities of thousands of people, and then two or three years later explorers coming by and finding nothing but ruins.

That seems much more of the type of collapse that would just leave entire buildings empty and abandoned.