Some of you are way too sensitive. This post wasn't even bashing Ayu.
Anyway, I think mostly why her sales have dropped, like already mentioned, is because Japan is tired of her. Sad, but true. She's not selling millions of albums because she doesn't have a fanbase that big. I doubt many people do. She use to sell over a million because she had casual listeners buying her albums and singles, but they aren't interested anymore. If one of those casual listeners from before happens to like a song from her nowadays, they'll most likely just download it.