They usually send out a message to the asker to select an MHO after a period of time. I try to keep up with those.
My thoughts are that it might be less active users who ask a question, then forget about their GAG account. But it may also be users who just don't care for selecting a MHO.
I used to be on Yahoo Answers and after a while if an answer was not selected as Best Answer, other users could vote on opinions. Then after a week or so the answer with the most votes got Best Answer. I think a system like that is needed here to help users who spend time answering questions to get points :)
GAG thought about this and has instituted an automated process of choosing an MHO after a specified period of time :)
1. Guess there are a few that stop using GAG once their query has been satiated. 2. The become irregular unless they have another query 3. There are other pressures that keep them away from GAG 4. They don't see an opinion that suits what they are looking for etc