Haha, 90% of the questions I've been a part of have no B/A or M/H.
I guess there's the obvious, or people forget... a lot. =D
Most Helpful Girl
Some people seem to forget, other people seem to ask a question and then just never come back...
Other people give a Best Answer almost immediately, and some take a bit longer.
Plus if a QA is unsatisfied with all the answers then they might just not bother picking one at all. Or sometimes there are multiple really good responses and it's kind of tough to pick just one.
I think they're trying to encourage awarding BA/MH by changing it so that you earn some Xper now when you make the choice, although some people still seem unaware of that change.1