Simply put, it's because Sylveon can do anything that Florges can do but better. Their stats are pretty similar, but Sylveon's higher base HP lets it take phyiscal attacks better and special attacks about as well as Florges despite having lower Special Defense than it. This makes it the better Wish passer as well. The biggest difference is in its ability, Pixilate, which lets it hit much harder. The only niche Florges has over Sylveon is Defog, but that isn't seen as enough of a reason to justify using it leading it to drop to RU.