No-one really knows and scholar experts of Pre-Columbian civilizations have debated this issue since the 19th century and the debate still rages on. Some kind of consensus seems to be forming around a series of factors of which the main three are:
Environmental unviability due to overpopulation and land overuse: The population was too large to be sustained by their agricultural systems which led all urban centers to collapse.
Political divisions: The Mayan Civilization was divided into sovereign city-states that were constantly waging war to each other. This led to civil strife and prolonged institutional stability that ultimately caused the system to collapse.
Environmental Catastrophe; a catastrophic drought that lasted for decades possibly depleted their water and food reserves to the breaking point.