Why did the people (occupants) leave Chaco Canyon?

Although it is, of course, impossible to know for sure why
pre-literate people did things, scholars believe they know why Chaco was abandoned.  The
basic theory is that climatic change and a decrease in the amount of resources that were
available nearby led to a gradual abandonment of the dwellings in Chaco
Canyon.

Beginning in 1130 AD, a severe drought hit the
region.  This drought lasted a long time and even after the worst was over, rainfall
remained below the previous average for decades.  After 1130, no more building was done
in the canyon.  Scholars believe that the drought and the decline in resource
availability made Chaco less viable as a major center.  Over the decades, the population
declined as people moved away to places that had more resources.