Primarily because there is no candidate in the state strong enough to pull it off. Secondarily, any candidate wanting to run for the Senate will challenge Cornyn, who has less backbone than a jellyfish and (to quote Mel Brooks) "no balls at all". Cornyn has never had a real job, and like all politicians without real world job experience, is worthless as tits on a boar hog.

Of the higher profile Republicans in the state, the Governor isn't strong enough to withstand a "solid" Democrat and the Lt Governor would never challenge Ted, but might go after Cornyn.

That brings you down to Congressional District 7, which has been occupied by only two people in the last 35+ years. While it may be the strongest and wealthiest Republican district in the state, Bill Archer never introduced a single piece of legislation in something like 28 years, and Culberson isn't any better, although he'll get 95% of the vote.

A selection amongst elected republican officials in Texas would be very difficult, but made easier by the even weaker selection of Democrat possibilities.