Four years ago, the best Democratic candidates didn’t run. Who can blame them? Why go up against an incumbent when you can just wait four years and not? Especially when there seems to be no heir apparent emerging.
I don’t like a system that discourages the opposition from putting it’s best candidate forward. I’m not sure how you could really fix that, though.
Of course, part of the problem may simply be the political climate in which a failed bid for the job effectively disqualifies you from getting your party’s nomination again.