A good question, but it usually bold down to cost of labor, land rights, and the EPA sanctions in the areas (this is why lots of Semiconductor manufacturers moved out of California years ago, Silicon Valley used to be known for the Silicon made there, now it is mainly a design and engineering think tank due in part to the strict California emission laws and regulations as well as land and labor costs).
Also those floods at the severity we saw are not a common occurrence in the hard hit areas. With the way the weather and other nat cats were last year globally no place was actually safe for one reason or the other.