You really aren't giving enough credit to the other candidates. Running for President is truly difficult and Santorum has been running literally for years. Even before Ron Paul was running Santorum was asking people for their support in New Hampshire and in Iowa. Gingrich is a bit of an egomaniac who has really hurt himself with many members of the elite (both supporters of Romney, of Santorum, and those who don't really care about any of the candidates but just want the race over), so he hasn't helped himself in any way, shape, or form.
And you have to remember that if Santorum won Michigan he could well have become the front-runner. That was the closest anyone got to actually defeating Romney. Finally, this primary process has been on the whole horrible for Romney- had Santorum dropped out after Nevada the race would have been long over by now, as Romney had serious momentum and a damaged front-rival in Newt to contend with.