it's always interesting hearing non-japanese people's opinions on ayu's "best" music, because it usually goes towards her rockier songs and/or more "western" sounding songs, usually not including her ballads. While people in Japan think of ayu as a "ballad" queen more than anything. (though, my general image of ayu is "remix" hahahah, but not so much anymore
)
If you ask Japanese people what songs they like by ayu, it's usually voyage, seasons, heaven, etc. The chances of people saying ourselves, because of you, rule, etc. are so low.
also, many people in japan think of ayu in terms of fall/winter music rather than summer.
These factors could play into the whole low sales thing.
but i just feel like it's not that great a single. therefore, people aren't buying it. just because it's a "summer" song doesn't mean it's good, you know?