Why? Your comment denotes a false dichotomy between "purism" on one hand, and having absolutely no standards of any kind on the other. Neither is an option. It's certainly possible to desire candidates whose records conform to movement conservatism, even though they may not be completely, 100% pure by any single FReepers own personal standards.
Cain, Gingrich, Bachmann, Perry - each has some flaws, but each also generally conforms to the standards of movement conservatism.
Santorum was not included in this category because he strays much further afield than any of them ever did. He has a legislative record replete with big-government votes and general conformity to the GOP-e, RINO mentality. In short, he is not a movement conservative. It's not just that he may not be 100% pure and has strayed off the golden path every once in a while, but that he is genuinely not a true conservative, he's a half-conservative at best, with us on some, main social, issues, but otherwise more at home and heart in Washington DC than in "flyover country."
You might want to go to the famous set of links that detailed Santorum’s votes. You might surprise yourself. Some things he voted down were attached to such wondrous goodies as SCHIP (remember that?). One of the border votes he voted down was in favor of a stronger one introduced by McCain. Of course, after the first amendment failed, McCain pulled his.
Yes, there were some party line votes. It pays to remember this was also back in the day when we had a weak GOP President and when passing a budget was a requirement.