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.