>>52772121To be fair, they are not REALLY "legendary", it's hard to explain. The way the game do things, for something to be a legendary, it's usually because they have some form of lore that etch itself into the region, creating its culture. There are some exception that make a person question Gamefreak logic considering some legendaries, (Type: Null, Urshifu). Now for the problem with the Koraidon and Miraidon have to be that they don't really have lore or significant part (until the final chapter) in the story. Hell, almost no one knows who they are.
I'll say, the reason Koraidon and Miraidon are considered legendary would have to come from their lore of Heath and how he describe the Winged King and Iron Serpent. The Winged King being strong enough to causes earthquake and splitting the land in half, while the Iron Serpent is powerful enough to turn the land into ash with lightning. And remember, there is also a third legendary Pokémon that have yet been reveal. One that is the reason why the paradox pokemon even exist. I'll say the reason why there is only two Koraidon and Miraidon would have come from the fact of their power. The third legendary could only create two due to it trying to make them true to their own lore and could only make two possible because itself is trapped and weaken while the other paradox are not as powerful as the Raidon, causing it to create as many.
TL;DR Third legendary make fake pokemon into reality while giving them the power of legendaries.