She doesn't appeal to younger or new fans. She's basically relying on the remaining 5-7k members of her fanclub that she had since the beginning of her career. Same problem in Korea (although on a much smaller scale). She's not earning new fans with her new releases and hasn't been since maybe 2007 (for Japan).
As for why she's not earning new fans, one explanation is that Japan is more focused on groups these days, rather than female solos. In addition to that, there are a bunch of other factors: BoA's TV performances, promotion, etc.