This is something that I fail to understand - why are there so many people there? Is there unobtainium in the hills that can only be mined from the region? Prior to modern air conditioning, why would people setup there when there are seemingly more hospitable areas?

People did prefer living in colder areas before AC.

The arrival of AC meant there was suddenly a lot of very cheap land around cities that are extremely unpleasant without AC. This resulted in migration and a housing boom which drives up the economy attracting more people.

Suburbia requires land and desert land is very cheap when market-priced, so long as the true costs of drinking and lawn grass watering for the land are externalized. Unfortunately, that leaves the region exposed to the Colorado watershed collapse and associated “which metropolis gets the axe” politics:

America’s desert metropolis Is on the short list to lose water (6 hours ago) https://www.nytimes.com/2026/08/23/us/arizona-canals-colorad...

(I suspect the interstate highway system also contributed to migrations, but haven’t done the research.)

Large silver and turquoise mines, basically.

Cheap land