Could it be that a lot of moderate dems crossed party lines? Could it be because Hoeffle didn't have a lot of money to campaign with? Could it be that the crat party figured that Spector was unbeatable and didn't mount a real push?
As far as Santorum goes I don't think he'll be able to be re-elected. There is a big block of left wing dems out east, a large group of embittered conservatives in the center of the state that are being urged to hold back their support of Rick because he supported Spector.
As for Santorum in 2006, I think he wins again. If the Demos couldn't beat him with a Presidential election at the top of their ticket (2000), there's no way they get him in an off year. Witness FL 2002: the GOP turned out enough Jeb Bush voters to get within 100K of their 2002 total, while the Demo turnout fell by 800K. And that was a state the Dems targeted for revenge.