The last few years have been defined politically by the Republican Party's breakneck run to the right. While Mitt Romney was considered the "conservative alternative" to the eventual nominee, John McCain. He has continued moving to the right in the intervening four years, but he now finds himself portrayed as a "Massachusetts moderate". The rightward move hasn't been limited to Republicans, such as Obama adopting McCain's "all of the above" energy policy.
Romney now finds himself locked in a protracted struggle for the Republican nomination with Rick Santorum. In 2010, GOP voters nominated a series of poorly organized, extremely conservative candidates for major office like Christine "I'm not a witch" O'Donnell and Sharron "2nd amendment remedies" Angle because they were the most conservative candidates in the primaries, even if they had huge shortcomings as candidates. Candidates who failed to toe the hard-right line were beaten in primaries, and incumbents learned that being perceived as a compromiser would lead to a primary challenge.
In this environment, Romney finds himself matched against Rick Santorum, who pretty much defines hardcore conservativism, at least in the pre-Tea Party era. Without Santorum's glaring weaknesses as a candidate, Santorum would run away with the nomination. This is an electorate that nominated Angle and O'Donnell, after all.
That Romney appears on the way to winning the nomination represents GOP voters setting a limit to how much they value extreme conservativism. While they still appear to prefer Santorum's principles, they recognize that a guy who can't even file complete delegate slates in big primary states won't be able to beat Obama. This is a level of political awareness they didn't seem to have in 2010 where their extreme candidates cost them control of the Senate.
So while Romney is clearly a superior and more-electable candidate, he has to fight Santorum on the latter's ideological turf. Despite being the more moderate candidate, he is winning. In the Tea Party version of the GOP. So it's not surprising that he's not storming his way to victory; that he's winning at all is impressive, given the makeup of the electorate.