What reason do social conservatives in Texas give for wanting to downplay Thomas Jefferson's role in U.S. History?
I agree that what the Texas Board of Education has done to the state history curriculum with regard to Jefferson is not exactly in line with traditional "conservative" thought. It is, however, pretty much in line with social conservative thought. But since you are asking specifically about the reasoning given for the change by the Texas Board itself, a little research reveals some quotes given to news organizations by Cynthia Dunbar, a Board member.
She claims the reason Jefferson's name was removed from a list of world revolutionaries in state guiding questions was because:
"The Enlightenment was not the only philosophy on which these revolutions were based"
Since his name was replaced with religious figures like Thomas Aquinas and John Calvin, we can also probably assume the Board members (at least, 10 out of 15, anyway) did not like Jefferson's embrace of the Separation of Church and State, which, it should be noted, is also a conservative value of limited government.
Outside of the previous discussion about White Conservatives, I would probably have to differ with the nature of the question. Those who are committed to the idea of Conservative thought woudl flock to Jefferson. His idea of laissez faire government that sought to reduce its role, the fact that he believed in a strict interpretation of the Constitution, and the idea that he desired to rest more power in the hands of local governments than the federal are all tenets of Conservativism. Perhaps, there might be some level of unease about how Jefferson went against his own political leanings with the Louisiana Purchase and didn't consult Congress in the process, or the fact that he ended up hurting industry with the various embargos which led up to the War of 1812. Yet, I think that Conservative thinkers would like much of what Jefferson argued and in what he believed.