See the primary results to date here.
The New York Times here suggests that pressure was mounting from evangelicals for Santorum to quit in order to unite the party.
Newt Gingrich and Ron Paul remain in the race but cannot muster enough delegates in the remaining contests to deprive Romney of the nomination.
Folding like this before the contest in his home state of Pennsylvania suggests that Santorum knew he would suffer an embarrassing loss there. One can imagine not wanting to have to explain why Pennsylvania voters decided to reject him like they did in 2006 and have to quit the race in the wake of that anyway.
