[bombcar]

> Over 200 underground cities at least two levels deep have been discovered in the area between Kayseri and Nevşehir, with around 40 of those having at least three levels.

I suspect that at least part of it was the ground in the area being easy to dig into to make a dwelling.

Dry underground dwellings are very good in general if they can be made easily.

[pmontra]

Non only the ease of digging but also it being self-sustaining: it doesn't need reinforcements in roofs and walls. In modern times there is something similar in Coober Pedy, Australia.