Lots of theories, rumors and plain old speculation in this thread. No one knows for sure. I seriously doubt Donovan was as unhappy as some are alluding here, but I do think he was feeling the impact of years, to the point of possibly feeling stale. Not the first time I've said that, nor should it come as a shock to anyone who's been around the block a few times. Nineteens years is a long time to spend anywhere, let alone at a football factory trying to build your own legacy, for which Donovan succeeded beyond everyone's wildest imagination. He can leave with no regrets, and that's an important consideration here.
As for the impact of the suspended O'Dome renovations, I think it was icing on the cake rather than a precipitating factor. Donovan's probably known for three or four years that he wanted to try the NBA again. IIRC there was a five-year moratorium on his seeking NBA employment again after the 2007 fiasco, and I think the successive Elite Eight appearances coupled with the Final Four berth a year ago with a team that so epitomized what he was all about helped renew him somewhat, just not enough.
At age 50, and with the right opportunity knocking at his door, the timing was right for him to move. It wasn't out of spite -- he spoke of the difficulty in leaving behind the Gainesville experience and the many, many meaningful relationships he'd built over time -- but rather because this is where he wanted to be.
