Whether they do that because they are teabaggers themselves, or whether they are afraid to cross the teabaggers does not matter -- because if you campaign and vote as a teabagger, then for all intents and purposes you are a teabagger. The truth is that an "establishment" (or somewhat moderate) Republican could not survive a GOP primary in Texas, because the teabaggers control who gets elected to the Republican ticket.
This is illustrated by the chart above from the Rasmussen Poll (done on March 3rd and 4th of 500 likely voters in Texas, with a margin of error of 4.5 points). Note that the favorability rating of teabaggers in Texas is a whopping 39% -- far above the national favorability of teabaggers, and also above the percentage of Texans who self-identify as being Republican. That means the huge majority of Republicans in Texas are teabaggers.
Add to this the fact that slightly less than 10% of registered voters in Texas voted in the Republican Party primary this year, and the fact that teabaggers are an energized group who tend to vote regularly, and it's easy to see why only teabagger candidates can get elected in a Texas Republican primary. Anyone who tries to run as a moderate will not only be defeated, but will probably be run out of the party.
If you find this hard to believe, just pick a Texas GOP elected official (any of them) -- then check their campaign rhetoric and voting record. They're all teabaggers. They couldn't have gotten elected if they weren't. The teabaggers are in firm control of the state party, and everything it does.