Well I think there are several factors contributing to this factor. First and foremost, they are not coming up with new types of events to keep things fresh. It becomes stagnant if you don't trying at least something different every now and then for most people. Only the most hardcore of dedicated will remain. Second, they introduced new mechanics but they way they implemented was likely a turn off to many players. Thirdly, the popularity polls probably upset many people (like yours truly) because previous winners were not removed from the next year's pool of contestants. This led to like, what is it now? Ten damn different versions of Anemone. I can imagine many people were like me, upset that Red Ginger was passed over in the polls or even in general consideration, convinced their waifu will never see an alt version. So why bother continuing? I think these are just a few things that helped contribute to the decline.