Do you know about uber legendaries? Do you know about them being overpowered? Well, the battle maison and battle institute don't allow those kind of overpowered pokémon. Here's why: These buildings are made to test your skill for competitive battle, not beat up your opponents with over powered pokémon. Some legendaries are allowed, mainly because they don't have ultimate stats, terrible abilities, etc. Do you think you could use a primal groudon, arceus, or a mega mewtwo in competitive? Not really. To answer your question, as I think I went off track a little bit (more like a lot, right?), some legendaries aren't allowed because of their god-like stats and they would make a pokémon battling test easy. Kind of like taking a test in school, with an answer key next to you. GAH! I forgot phione. You know the legendary that really stinks? Yeah, him. I have no clue why he's banned, but it's game freak. Hope I helped!