You'd think that this might be because of Norton's "hard to work with" status, and previous rumors of his final cut war with Marvel. Turns out, that is not the case. The WGA simply ruled that the rewrites weren't substantial enough to include Norton along with Penn as "writer." IESB broke the story and says this...
The IESB contacted Zak Penn’s office to ask about the credit. They confirmed Penn will be receiving sole writing credit on the film. Universal reps are saying, “WGA determined the writing credit not Zak or Edward or Universal or Marvel. WGA always determines final writing credit for our films right before the film comes out. Up until that time, we include the writers on the project to date.” A call to the WGA got this response, “Zak Penn is receiving sole credit for story and screenplay.”
Sounds to me like Norton is getting screwed by the WGA for being multi-talented.