Maybe he wants to wait for a west coast team or large market team to bottom out, rather than be drafted by a certain team. There have been players who have refused to report to certain teams (Manning, Elway come to mind) when selected first overall. Staying in school would help him avoid that controversy, if he doesn't want to go to the team that is likely to select him.
Also, if there is a new CBA in place, it very likely will have a rookie cap. He might wait a year for that cap to go up.
Finally, it's been said elsewhere on this board that he takes his studies seriously and that his family doesn't exactly need the money.
I think (and hope) he'll declare, but there are some reasons why he wouldn't.