I found multiple questions about this within several last year:

http://meta.stackoverflow.com/questions/144952/how-to-search-only-unanswered-questions-with-an-open-bounty

http://meta.stackoverflow.com/questions/54476/how-can-i-search-for-bounty-questions/54477#54477

http://meta.stackoverflow.com/questions/108696/list-previously-featured-questions

http://meta.stackoverflow.com/questions/68253/advanced-search-criteria-for-bounty-questions

http://meta.stackoverflow.com/questions/71079/advanced-search-option-to-include-questions-with-bounty-among-the-search-criteri

http://meta.stackoverflow.com/questions/130252/can-we-get-some-better-bounty-hunting-tools

Most of answers boiled down to two ideas:

• Use hasnotice:1 answers:0 closed:0. It's imperfect, but what can we do...
• Go and browse featured manually or with limited filtering options

Why it was never implemented? It sounds like reasonably straightforward thing (and without any obvious downsides)