Maybe nothing 'changed'. Maybe EA just didn't like the way Wii U was panning out prior to release. Too casual, too low of specs, too who-knows-what. Nintendo and EA's philosophies never matched up very well.
But as others said, I'd be surprised if EA completely blackballed the system. Sales for Wii U honestly have to pick up when there are actually games for it, if it can sell like this when there is practically nothing unique out there for it. And when they do, EA will jump on board.