I think the reason is that the original setting of Vulcan island disturbed the storyline, especially Hoenn and Sinnoh. After entering the centre of Vulcan island, we can use lv100 pokemons from Kanto or Johto before we finish our journey. We don't need to train our pokemons (or buy a pokemon) to fight Rayquaza, Deoxys, Giratina and some gym leaders. Compare to the other players, this is somehow unfair.