^ as you see in some of the responses in this thread alone, life goes on for some of them. either they quit/got tired being part of the mainstream industry and went indie, or real life catches up with them (either they got married and quit, or they have babies and quit, or they have personal issues to deal with, ie family issues, illness, etc), or they are still trying to get signed, or they focus on other aspects of the entertainment industry ie film and tv programs, or went the songwriter road. it's not just JPOP mind you. other music industry are affected as well.
oh, and if there are news about MELL, try checking out the I've Sound Artist Thread: http://jpopcentral.yuku.com/topic/105/