I believe it is becuase it isn't for doubles. Look at singles it's nu. Look at doubles it's ou. It's likely because of the fact that it has not very good support in singles. It's main boost is tailwind and other support moves (sunny day, charm, cotten spore, etc.) In singles, tailwind only has 2 or 3 turns after you set it and switch out. On the other hand, in doubles, it's instantly in effect with the other Pokemon on the field, and then it can switch if you want to have 2 Pokemon for the boost. So, basically it's because it was banned for vgc, a doubles format, and you were looking at its singles teir instead