Like all those mythicals that don't even have 600 base stats, why are they banned, but you can bring a 700 base stat Mega Garchomp or Salamence?
I think they wanted to make sure that people who attended events can't possibly have any advantage over the people who didn't, so they banned everything that was event exclusive even if it's not obvious how they can bring an advantage.
By the way, you're not supposed to select or up-vote answers if you don't like them.