1) People just don't like Mitt: The entire GOP primary process so far has consisted of Republican voters desperately trying to find an alternative to Mitt Romney. Doesn't it say something that GOP primary voters have, at one time or another, preferred Donald Trump, Michele Bachmann, Rick Perry, Newt Gingrich, and now even Ron Paul (In Iowa) to Mitt Romney?
To some people, this is a plus. They think that if conservatives don't like Mitt Romney, that means moderates will like him. This misunderstands how the process of attracting independent voters works in a presidential race. While it's true the swayable moderates don't want to support a candidate they view as an extremist, they also don't just automatically gravitate towards the most "moderate" candidate. To the contrary, independent voters tend to be moved by the excitement of the candidate's base (See John McCain vs. Barack Obama for an example of how this works). This is how a very conservative candidate like Ronald Reagan could win landslide victories. He avoided being labeled an extremist as Goldwater was; yet his supporters were incredibly enthusiastic and moderates responded to it.
Let's be perfectly honest: Mitt Romney excites no one except for Mormons, political consultants, and Jennifer Rubin. To everybody else on the right, Mitt Romney vs. Barack Obama would be a "lesser of two evils" election where we'd grudgingly back Mitt because we wouldn’t lose as badly with him in the White House as we would with Obama. That's not the sort of thing that gets people fired up to make phone calls, canvass neighborhoods, or even put up "I heart Mitt" signs in their yards.