Answer:
Very little rainfall occurred, and then a long drought had started. For this reason, people of Chaco Canyon (Anasazi) left that place and never came back.
Explanation:
Scientists have found one possible cause by dendrochronology (the study of looking at tree rings) in the Chaco Canyon area. In the period between A.D. 1125 and 1180, very little rain fell in the Chaco Canyon. After 1180, rainfall briefly returned to normal. From 1270 to 1274, there was another long drought, followed by another period of average rainfall. In 1275, yet another drought began. This one lasted 14 years.
When the people of Chaco Canyon (Anasazi) started facing this little rainfall, they fall into water problems. Their problem started to solve when the rain became typical, but they again started facing the same problem when long draught had begun.
So, finally, they (Anasazi) decided to leave Chaco Canyon and never thought about the return back there.