One reason, as another poster suggested, is that many of the questions are posted by sporadic visitors who are not invested in the site and do not log in frequently enough to bother awarding answers.
Another reason is that the traffic to that site has gone way down and hardly anyone bothers asking or answering anymore. The site offers some revenue share, but the amount is so piddly and you only get it on questions where you are awarded best answer. I think most people have just given up on using that site so most of the questions and answers have gone abandoned. That site also has a high percentage of unanswered questions.