Because of the last month of total slime warfare against Obama. Nagourney:
For Mr. Obama, the outcome came after a brutal period in which he was on the defensive over the inflammatory comments of his former pastor. That he was able, at a minimum, to hold his own under those circumstances should allow him to make a case that he has proved his resilience in the face of questions about race, values and patriotism the very kinds of issues that the Clinton campaign has suggested would leave him vulnerable in the general election.
Wright is a grenade that will fizzle. The right will try other gambits - the Ayers crap and if that doesn't work, look for them to take aim at Obama's wife. But Obama's survival - or rather the voters' refusal to make this election about the Freak Show - suggests that Newt is right. This will not work this year.