I have a few guesses about the loss of popularity.
Possible Answer #1: Sanae is losing ground because most Sanae fans aren't very dedicated and are quick to like other characters.
Possible Answer #2: Dedicated Sanae haters are becoming more numerous as more logical candidates for player characters are overlooked in her favor.
Possible Answer #3: Japan is very fickle and quick to change their opinion about nearly anything.
Possible Answer #4: Sanae fans may not as vocal as they used to be, or perhaps fans of other characters are becoming more vocal.
Possible Answer #5: Remi changed Sanae's fate to make her less popular.
I don't have any idea why she's becoming more popular in the west however. Maybe we're just stupid or something. Doesn't bother me either way though as Sanae isn't very high on my list.