[ghaff]

It's a combination of the license and the fact that it's Oracle, of all entities, that owns the copyright. Perhaps either one by itself wouldn't be a dealbreaker but the combination is. And, of course, Oracle could have changed the license at any time after buying Sun.

(Of course, Jobs may have just decided he didn't want to depend on someone else for the MacOS filesystem in any case.)

ADDED: And as others noted, there were also some storage patent-related issues with Sun. So just a lot of potential complications.