Land in Texas and Arizona *is* rather cheap.
They already have fabs nearby, and so, fab suppliers/services in the area.
They have a large employee pool in the vicinity, with modest salary expectations.
The southern US mostly doesn't have blizzards and ice that regularly knocks out electric supply lines, and/or makes it impossible for employees to get to/from work for days/weeks at a time.
Water is about the cheapest resource around. Even in water-stressed areas (while it may cost several times what it would in wetter parts of the US) it's still an insignificant expense for any industry that doesn't need an absolutely obscene continual supply.
Roof-top PV solar power works out a lot better in the southern US than further north.