Good question. Maybe they got the answer they needed and didn't feel the need to stick around to tick any of our boxes? Member for 4+ years with 156 rep - even as stingy with votes as we are, that doesn't strike me as a user that's terribly invested in SF in general.
Bounties are kinda strange... as much as I like collecting free internet points as much as the next geek, I've never tried harder to answer a question because of one. I've either had a useful answer to someone's question and the time to type it up, or I haven't.