Not so much that he's Catholic. Is that he doesn't have the southern/country/low class vibe that the GOP desperately needs. Romney already has the grown up college Republican vote. Especially in Virginia, North Carolina, Florida, and Ohio he's gonna need red meat Take Back Our (White) Country crowd. Instead, he went for the best looking Ayn Rand fan in America. I've yet to see Ryan campaign, but I don't get the impression that he can get the job done.
Either the "white working class" vote is more Democratic (or less Republican, alternatively) than the pundits give them credit for, or the Republicans take them completely for granted and really don't care about them.