I think there's a couple factors at play regarding where they play their shows. The biggest one is the self-imposed limitation that they put on themselves not to be away from home for more than roughly 5 weeks at a time. Were they to remove that restriction, I'd bet that they could more easily play a lot more of the US and Canada for that matter. Maybe now with most of their kids being in college or nearing the end of their high school years, perhaps that will happen. Then again, maybe not (see Endless Sacrifice lyrics). But with this restriction, I'd imagine that they want to get as much bang for their buck as possible.
So it's not just a matter of individual cities and whether the promoters are willing to front the money the band requires, as I'm sure promoters in at least 1 or 2 Texan cities and Atlanta probably would be happy to bring DT to their town. Really, it's about planning a tour to wind thru the US and Canada so that they only have a limited number of days off (which the guys do need, especially JL), play as many shows as possible that are going to reap the biggest rewards and limit the travel/transportation expenses as much as possible. So I'd bet that's the reason why DT typically plays the SW coast, makes their way east up thru the midwest and ends their tour on the E/NE coast, and why the NW and SE parts of the US are more likely to be skipped.