As I've said time and time and time and time and time again, part of it is many of the old-timers who really breathed life into this place back in the old days are simply not here anymore: many of them have moved on, they've got lives, they've got jobs, they've got families, they've got little time to devote to MC let alone the internet, and then there are some who feel MC engages in too much "high school drama" atmosphere, and others feel MC doesn't embrace fan input as much as it used it (hence the main reason why Prawnie launched the Mindset years ago, because MC used to host fan-written articles, reviews, and such).
Since then, we always have groups of newbies who come and go: nobody really sticks around much anymore, and a lot of it has to do with people go through phases in their fandoms.