>>46645282A pokemon is legendary because gamefreak says it is. they used to have some higher importance to creating the world in 99% of cases, even if its a bit vague, or were one of a kind (or a very limited number) but the definition kept expanding. It was a bit of a strange thing in the first place since you know, the regular animals are already magical elementals, how is Moltres the phoenix or Zapdos the thunderbird special? well, they sit in a dungeon, thats why, but the movie gave them some importance at least. legendaries need important lore to stand out among the regulars.
And even then a lot of regulars could have easily been legends. Arcanine, Ninetails, Spiritomb, Unown, Dragonite, so on. not to mention sprout tower, the most important legendary location. and a lot of legends could have just not been like Meloetta, Zarude, Kubfu, Latios/as, Diancie, etc. Its messy.
I think they should really have made sure they were all one of a kind canonically, even if you can technically have several in the games.