It's so weird how illogical people are when they're on the hunt for power...
1) It's pretty much impossible to win when the tide of the country (or at least its angriest voters) are moving in the opposite direction from your own party. Any Republican running for President in 2008 just seemed like a moron to me since Bush and his party had record low approval ratings. There's no way that deficit could be overcome. Kucinich would have eeked out a narrow victory over McCain.
2) If you're running in your party's primary and you noticeably hit a peak and then start plummeting down in the polling, it's over! Voters don't consider voting for a candidate en masse, back out, and then reallign with that candidate. Small fluctuations are a given, but not large ones. If you're somebody like Bacchman or Perry who were in the 20-30% polling range and then plummeted to single digits, you'd be wise to get out sooner than later. People fled from your camp for a reason. By this logic, Cain is pretty much doomed as well since these sexual allegations and his lack of adding anything new to the discussion since he came out with the 9-9-9 plan and will therefore lead to a drop.
The only non-Romney that should stay in the race is Gingrich. He has not had a peak and drop-off yet and therefore is the only one in the race that can presumably gain momentum and try and top Romney. People like Bachmann, Perry and Cain that continue to run only do so because they saw their name atop the polls and can't get the image of winning the nomination out of their heads. If any of those three had been wallowing in low digits for the entirety of their run, they'd probably have fled by this point.
Sure enough, this will be a two-pony race after Iowa and probably done not much longer after that. It will be the "John Kerry effect" again where winning the early primaries will steamroll everyone else.