The Mayans abandoned their cities around 800 C.E. From my perspective, there does not seem to be a lot of evidence why. Some theories include Environmental degradation and overuse of land, political dissension and social unrest, natural disaster, and outside invaders. However, these theories remain a source of debate.
What were the factor(s) and underlying reasons that primarily caused the Mayans to abandon their cities?
There's always room for discussion, but perhaps the section The Maya from our FAQ will answer your inquiry.