There are many theories related to why the Anasazi disappeared, but there is no solid evidence that gives one specific reason. It was previously suspected that the drought in the late 1200s drove them from their longtime homes, but more discoveries have shown that this would not have been enough on its own to drive them out.
The idea that these people totally disappeared is a misnomer as well. The modern day Pueblo Indians are direct descendants of this race of people, so they never truly disappeared. Evidence shows that the Anasazi may have chosen to leave due to being mired in the area for too long and the land being used to its limits.