I've heard alot of crazy speculation as to why she sold so low.
One said that it was because of the Kago scandal. (Wtf?) ANd another said that it was because of JunJun and LinLin (LOL).
The theory that made the most sense to me though, was the one that said that H!P became complacent with releasing ballads, after the surprising success of Aruiteru. Which was why Berryz had Very Beaty, and Maki got Secret. Ruining both of their runs of strong single after strong single. But Aruiteru was an exception I'm guessing, because ballads usually sell like crap, and they should know that by now.