Bush Sr. was from Texas, and the economy was good (while Reagan was President and Bush Sr. was VP, thus allowing the GOP to take credit for it).
Bush Sr. was not a true Texan the way Ann Richards was.
The good economy should still allow him to squeak by in Texas. I already think that Texas was trending Republican in Presidential elections at this time, and I don't see why this trend would stop, even if Richards was Governor there.
It wasn't THAT far removed from Democratic leanings, and Ann Richards was very popular until Dubya came along. I think that if she was that popular there in 88 it wouldn't have been that difficult for her to pick up Texas.
