The fact he was created to be the ultimate, having only one in existence, might just have something to do with it.
If you want to really ask that question, why are the legendary beasts legendary, when supposively they were created from common Pokemon due to Ho-Oh's power, and supposively more than one has existed. --- Only insecure and ignorant people try to debase someone online based on their username.
But no one even knows it exists other than Red and the scientists who helped birth him.
The same started with Mew, Mew was so rare it was considered to be a mirage because no one had literally seen them. it doesn't matter whether they were seen only that they are beyond rare, have uber Powers (Dialga/Palkia being idols of time and space), and or changing the world in some way (Regi) --- Only insecure and ignorant people try to debase someone online based on their username.
It's just the arbitrary "definition" of "legendary" that the "fans" give to it.
Same reason why Volcarona can't be legendary or pseudo-legendary: because it doesn't fit some made-up list of rules that for some reason is agreed upon by pretty much everyone. --- THAT'S MISS HOME ROWED TO YOU the truth is the truth still hurts, and it will just get worse
Perhaps it's legendary for its sheer power and capabilities? You're trying to apply too much logic on a basis which the game does not supplement too well towards. We'll just go with some of the gameplay aspects that consider it "legendary"
- High BST (Don't come at me with Slaking..) - Only one - Origin oriented - Presented as one of the strongest Pokemon in the entire world