He's too opinionated. I'm serious. Ever since Bork, the trend has been towards finding nominees who have left the smallest possible paper trail of potentially damning legal opinions. And so you're left with the farcical confirmation hearings of e.g. Elena Kagan. The majority of sitting justices are/were not brilliant jurists or legal scholars for this very reason.

I would add he is old enough that the appointment would have relatively limited value. In an increasingly politicized court, it might be better for a president to appoint someone with limited intellectual weight who will be a reliable (liberal or conservative) vote on the supreme court for a long time than a brilliant scholar who might step down in a few years, at least from a political power standpoint.