This has been tackled before very succinctly. While the state is seeing some modest change, the fact is Hispanics who settle in Texas (while still voting majority Democrat) have a smaller split between the parties than in other states. Other than perhaps Florida, Texas actually has probably the highest percentage of conservative Hispanic voters.
For all its faults (Cornyn), the Texas GOP is one of the better state parties. In fact, it ranks right up there in the top 5. The resistance to the grassroots is a lot weaker, and the candidates make very good cases for conservatism.
If the state parties of the nation were more like Texas, Illinois would be very competitive in a Midterm cycle, and Alaska would never elect scum like Begich.
Spit!
/johnny