There is still some dispute among professional archaeologists on this, but the evidence strongly suggests two main causes. One was internal warfare, much like that of the Greek city states, who ultimately were all conquered one at a time by Rome. For the Mayans, the internal warfare led to the destruction of many cities and the simple abandonment of others.
Combined with this was evironmental destruction as the land could not support the size population that developed. When warfare destroyed the canals, food sources vanished and the population crashed. People wound up living in small villages instead of magnificant urban complexes.
The Aztecs arrived a few centuries later, followed by the Spanish. However, much of the Yucatan and Chiapas still speaks Mayan dialects.
