Rick Santorum lost for precisely the reasons stated.
When he supported Arlen Specter, a man with a long and well documented history of pro-abortion activity, his principled base was immediately incensed---and lost. Santorum even made commercials for Specter saying "He's with us when it counts." He stabbed his base in the back and he made no apology for it; made no attempt to make amends.
That single act shook his supporters from their stupor and a subsequent review of his public voting record revealed that he really wasn't as conservative as we'd been led to believe. Fiscally, he was a disaster.
His conservative reputation was highly over rated and undeserved. Good riddance.