It was just a big song
Came out at the right time when she was actually popular. It's a ballad. Japan loves ballads.
It's a really emotional song too, and a sad song. And they just eat that up. I personally love love the intro that sets the whole tone of the song.
I also don't feel like it's as cheesy as say, Voyage, to contribute to that useless comparison.
And I agree with whoever said it up there, if SEASONS is overrated, then so are all of her other songs.