What happened to the ancient Puebloans in southwest Colorado?

During my trip to Colorado earlier this year, my host took me to the Colorado National Monument We hiked along the trails, aiming for a waterfall sourced from snow melt high up in the mountains.  Other amazing sites within the Mesa County area suitable for hiking include the The Little Bookcliffs and the regular Book Cliffs which are mostly shades of grey, extend into Utah, the same Book Cliffs that's targeted for increased drilling and mining, and give the appearance of constantly disintegrating from what seems to be sand running down the sides of the cliffs.

But I hear that's not so, as the soil on the sides is quite firm and sticks to the rocks.  Even so, it does not attract me the way the CNM attracts. And of course there's Arches

and Canyonlands but those are in Utah, about 2 hours away by fast highway.

On another trip it would be nice to go see the Little Book Cliffs as they seem less steep than the Book Cliffs and to see the wild horses there.  The Book Cliffs seem way too steep for my limited hiking skills and non-existence rock climbing abilities.

Also in the area around or near Grand Junction is the Grand Mesa and further to the southwest is Mesa Verde.
I did go snowshoeing in the national forest on the Grand Mesa

as it's so much higher and thus still had snow in April/May.

Getting to the spot where we got out to snowshoe involved a long drive up a very steep height

but even so we got only half way up, I was told, where we stopped and parked.
A few miles down the hill from where we went snowshoeing/hiking is the famous Powder Horn ski

resort or as they now label themselves the Powder Horn mountain resort, to include summer activities presumably. But there's lots to enjoy when there's snow still on the ground.

This is just one glimpse of the view on the way down the hill up to the forest.

Going to Mesa Verde would involve much more complication because of it being a a greater distance from Grand Junction than the other sites, along with more extensive hiking, climbing and exploring, and at greater heights, as entering and exiting ancient cliff dwellings come into play, including climbing up skinny ladders and climbing into holes in the floor to climb down ladders, presumably in the dark.

This photo is from the Mesa Verde FaceBook page for the National Parks Department, taken by one of the rangers there.

Much is said about the alleged mystery of the disappearance of the ancient Pueblo people who used to inhabit the cliffs dwelling.  For the first time, today, I encountered a theory of population collapse overburden in an article in Wall Street OTC which makes sense to me as an explanation for why the ancient Pueblo people left the area. Native American population collapse  There were too many inhabitants and they ran out of space and food, and most likely water.