Romney already had RELIGION PROBLEMS as a Mormon. Fundy whackadoodles were holding their noses and trying to make the best of voting for him anyway. Now he hits them with this Eddie Munster-looking Catholic car-bomb, known as Paul Ryan. In fairness, Romney didn't have a lot to choose from anyway. I suspect that many of the other candidates, such as Portman, or Pawlenty, or Jindal were starting to get gun-shy about hitching themselves to Romney's clown wagon.
Also, one wonders why he chose to do outside of prime time. Is his political team really this bad at PR? Why bury your biggest campaign decision. Friday by noon would have generated astronomically more coverage and you could still make the Sunday talk shows. If you do it on Saturday though, it would take some fancy footwork to get you on the Sunday morning circuit, which presumably is already booked.