None of the other candidates AFAIK have the organization and/or money to take it all the way. I also seem to recall that despite McCain's frontrunner status and the inevitability that surrounded his campaign in 2007-2008, he did go through a rough patch for awhile where he fired/replaced most of his campaign staff and he had to face down a strong challenge from Huckabee. Romney has no real incentive to drop out of the race yet. He can (and probably will) "retool" his campaign and come back strong. He just may not have as clear and easy path to the nomination as he (and most establishment Republicans) believed he would. With Cain, Bachmann, and now Perry gone, the socially conservative "not- Romneys" have fewer candidates to be divided among as well.