Urban renewal ping.
I think it’s a fairly simple demise. You’ve got a “surge” of some tribe....based mostly on domination of lesser tribes and not a combination of science, civilization and warfare. They run through all the resources within ten miles, then thirty, then fifty. Rather than live there in the “heartland” where culture means alot...most folks get to a point of moving further out...away from soil erosion and closer to real forests which haven’t been cut down. One day, the head Mayan dude wakes up and realizes that he has a vast empire but there within the ten mile stretch...it’s a lot less than five years ago. Rather than build a civilization which is the Rome tradition...the culture just kinda gave up and moved on.