Yeah, in the end Santorum was just too conservative for many of his constituents. I'm all for standing for your principles, but past a certain point you have to remember that you're in the Senate to represent every citizen in your entire state, not just yourself. If you're unwilling to compromise on some of the stronger stuff, then you'll have to accept the risk that you might get voted out next time around. (For the reverse side of the same coin, look at Ned Lamont in Connecticut, who was far too insane of a moonbat even for a deep blue state.)
Some states are just so liberal and so corrupt that more than 10% margin is needed to win an election.