Because it seems to be melting.
A new poll of New Hampshire Republican primary voters has Romney consolidating his lead while Giuliani and Thompson plummet. Apparently the Thompson buyer's remorse isn't just on the part of his House supporters, but NH voters who didn't formally endorse him are free to ditch him rather than just giving anonymous quotes to CQ Politics. Do you think Grandpa Fred lost his manly smell?
While Thompson's loss of support is part of a broader collapse, analysts don't seem very interested in talking about Giuliani's drop. It's true that his strategy is to bet the ranch on the February 5 primaries, but going into them having performed as weakly in Iowa and New Hampshire as it now seems he may could be a problem for him.
University of New Hampshire for WMUR (PDF). MoE 3.5%. 11/14-18 (9/14-24):
Romney 33 (25)
McCain 18 (18)
Giuliani 16 (24)
Paul 8 (4)
Huckabee 5 (3)
Thompson 4 (13)
Also noteworthy is that they'll be drawing on a voter pool with few independents in it. According to the same poll, 65% of independent voters plan to vote in the Democratic primary. Guess we know where the energy and broad appeal are this year.