A. Timing. Polls at this point are worthless. Polls against Obama at this point are worthless. What matters is in November.
B. Romney hasn't been truly vetted. Not like Perry, Cain, Newt, Bachmann. I know, neither has Obama either. Romney has a ton of negatives out there. Look up how many jobs lost under Bain Capital. He's the caricature of the evil corporate one.
BTW - November of 2012 that is.
This website is clear evidence for the fact that any current GOP candidate is not going to get his full number yet, because there are plenty of moron proclaiming that they refuse to vote if their candidate is not nominated.