He's conservative. He doesn't alienate the conservative base of the GOP, like Giuliani or McCain, or cause the to have doubts his sincerity, like Romney. He also seems to be fairly well known, unlike the rest of the second tier, GOP candidates.
Didn't mean that to sound as snarky as it probably sounded.
But Thompson is pretty much a blank slate. He needs to get out front and fill in the blanks. I know what most of the others stand for, for good or ill. I really don't know much about him, although maybe thats my own failing, I just haven't followed him in the news much.