Specifically no one knows. Beliefs are climatic changes or a long term draught caused the people of Mesa Verde to abandon the area and go elsewhere. They possibly joined or massed into other smaller groups that became the Southwest native Americans-like Comanche, Apache, Zia, Pueblo, etc.
Environmental changes made it harder for them to grow corn, which forced them to migrate south to join other Pueblo villages in Arizona and New Mexico.
Believe what people want but there is NO HARD EVIDENCE that environmental change was the ONLY CAUSE for them leaving what they had worked so hard to achieve, however it is was the major reason for eventually leaving the original sites. Neiboring tribes fighting over food and resources was an impact on the Pueblo Nation that eventually led them to their demise and leaving the valley completely where they once again built magnificent structures about 60 miles away. Eventually the process of continuous droughts led to the demise and abandondmend of these amazing places as well and recent archaeology has shown that the last days of the Pueblo people were one of many things we couldn't imagine.
On top of the mesas. They are the pueblo Indians, the Hopi, Zuni, etal...among other tribes
Were did the Indians go
Most migrated into the Colorado river valley. Some went down to Mexico. Ground imaging satellites have found the ancient trails between the south west and the Mayan civilizations of central and southern Mexico. Trading was known to occur by the amount of turquoise found with the Mayans. What the indians traded for is unknown but i suspect probably obsidian for tool making.
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