I put out a bounty recently and got zero comments and zero answers. It then expired.
I'd still like to get this question answered, and (if the feature existed) I would even go so far as to increase the bounty amount. But given that I just put 50 reputation towards getting no response at all, it seems like a waste to put an additional 100 reputation towards it and hope for the best.
Here's two feature ideas that might help in this scenario:
The ability to resurrect an expired bounty by adding more rep to it.
Getting bounty returned if there is zero response (no comments, no answers) to it.
Neither of these features seem particularly game-able, and would make it less of a risk to put bounties on uncommon issues.