The reason? Republicans believe what they say when talking about faith. Democrats worship power above all else!
Santorum's remarks on prenatal testing like amniocentesis, questions about Obama's political "theology" and old comments from 2008 about Satan have stirred up the left wing media. If Santorum is the nominee we can expect an endless number of questions and stories painting his views as extreme.
Never mind that Obama invokes the Bible in support of his tax increases or that his own political bible, Saul Alinsky's Rules for Radicals is dedicated to the Devil. Any critical questioning about Obama's faith, or the lack of it, will never be permitted to become an issue in the mainstream "news" media.
Santorum will have to be very careful to avoid stepping on the landmines of social issue controversy that the press will be only too happy to place in his way if he wishes to avoid becoming marginalized as a fringe candidate representing orthodox Catholic views. Remember the adage from Bill Clinton's first campaign "it's the economy stupid." Santorum will only hurt his cause if he spends any more time talking about birth control.
That being said, there is another reason that the media don't highlight what Obama and the Dems say about religion. It's because they don't take these statements seriously and they know that Obama doesn't take religion seriously either. The only thing that Dems believe in is the acquisition and use of power in the greater socialist cause. Religion will be used as a tool and discarded afterwards.