The main thing I think is the cause of low participation would be the fact that they forgot about it. Personally, I have forgotten to participate in several contests.
I am not sure about others but I personally can't participate in most contests because I have a busy job in real life and I am doing multiple things online as well that's the reason why I can't look for games and rounds board that often. I think only posting a prediction without proper research is no less than spamming that's another reason why I don't just post predictions in most events at a time to see if my luck works or not. I know sometimes luck works but as members of the forum we should only participate in events if we can give them some time and we can do some research about the teams before positing our prediction.
I get your point, it's almost pointless to participate in these contests when you don't have the time to do that but I think people forget the main point of these contests, it's normal to care about winning but it's all about entertainment, making a prediction doesn't have to be as difficult as rocket science, with the little knowledge that you have you can make a prediction if you are interested. "Emphasis on the word interest"i believe that if you are interested in contesting you would definitely find a way to make a prediction and mind you doing research increases increases your chances of winning but there are no guarantees.