1. EADS/Northrup is going to produce a better product for the price the U.S. government is going to pay.
2. See #1.
3.... In other words: LOW BIDDER.
EADS/Northrop may simply be producing a much better aircraft for slightly more money (and in light of the comments I've seen on various threads, this could very well be the case).
I'd also suggest that Boeing didn't do itself any favors by: (1) failing to meet deadlines in the delivery of other aircraft in recent months, and (2) having a couple of its executives involved in a scandal involving potential bribery of U.S. military and/or government officials in the months leading up to this selection.