I gave your question a Five Star cause I think it's a great question :)
This is not a question to GAG (who generally know the answers to almost everything shot at them - glad they have put a lot of thought into these things before making a rule) but to the users that makes a lot of sense to me :)
A lot of us ask questions only when we require to (obviously), get answers and then vamoose. Could be a lot of reasons for that. But I make it a point ot provide a rating (99.9% of the time) to each question and whenever I do ask a question I'd select a BA if there are more than one person answering it lol or if I see that the single answerer has made enough effort to answer me (ofcourse while making sense).
**There also may be quite a few who didn't find any from the tons of answers appropriate enough to suit their requirement which is why they don't select a BA.
But however, I've seen and known answers (by me and others) that can be a BA but haven't been selected.
I appreciate not only your question here but also the initiative.
Matter of fact I had thought of asking this question directly to GAG and also recommending that unless a question asker doesn't select the BA for a particular question after a certain number of days then he / she shouldn't be allowed to ask further till they don't but then again thought of what I've written in paragraph ** struck me. Still one can always select from what's available :)
Sometimes I do not because many of the answers are the same so there is no point to picking a best answer. Other times it is because none of them pop out to me as being good. Sometimes I forget that I asked the question, or it isn't needed anymore.