My pet theory is that Kennedy delivered the "catholic" vote in exchange for it.
The only 'catholics' who voted for Obama were those liberals for whom 'social justice' is the deciding factor. They look down their noses at Catholics who take the teachings of the Church seriously, if those teachings interfere with what they think is 'fair'. To those liberal Catholics, abortion is simply one issue among many, and they think it's silly to stress it over poverty, injustice, racism, and sins against the environment. So the fact that Obama was going to be the most pro-abortion President ever didn't bother them at all, because they hate Republicans.