There have been many articles written about this issue, and I have provided a few links for you. Generally, the conservatives who oppose any emphasis on Thomas Jefferson in the textbooks find it objectionable that he played a significant role in the separation of church and state. He was also a Deist, which means that he believed there is a God, but that he did not believe that any one particular religion was better than others or that any particular religion had a direct pipeline to God. (I have included a link on Deism, too.) This is offensive to conservatives, who believe that the United States was intended to be a nation founded on Christian principles. Can you see why a Deist who believed religion should play no part in government would be someone conservatives would want to de-emphasize?