[Corvina]

It's an overall phenomenon in the Japanese music market with some few exceptions (idols and male solos/bands mainly). Apart from Namie and Finally I don't know any female solo actually rising in sales.
People don't really spend money on music, more on going to concerts and buying merchandise, if at all.
I've even just read a comment on Kuu's fan corner on fb, that a fan wants to buy DH's 8, but doesn't have the money for that.

Plus remix albums never did too well. Kuu's first bunch was an exception, cause they were new for her probably (and Beach Mix cause there wasn't a single or album from her at that time).