It's hard to say why. Erdrick is a member of the actual Hero Class. However, it's probably because Hero is both a Class and referring to the story's role for the rest, but Luminary is also the default for promotional reasons(now why Luminary and XI got significantly more content is another story. The Stage, Kirby being based upon Luminary, and him being the default alt all make sense. But the rest is kind of sad. Erdrick is a big deal and is not treated as such. He at best was chosen for a spirit event and took over the trailer very clearly. Though the trailers are outsourced too, so we can't actually determine how much Sakurai or others could've influenced the choices behind stuff like that very well. DQ3 being biggest in Japan was probably all there is to it).
The original Protagonist is also a Mii costume(in practice), which makes it ironic that he's not playable despite Mob Smash saying the debut of the Hero Class is from his game.
Also, the Monster Spirits specifically use their designs from the game they debuted in, so they're meant to represent that game specifically. That's why they're listed "from that game". I'm not saying it's the best way to do it. They didn't even have to specify the game, but the series, if they wanted. However, they went with them being "a member of this game", so they clearly thought representing a specific game mattered. That said, this is probably because they were throwing a bunch of Games on the Switch and wanted to have more than one game very clearly referenced. That, and the Mii Costumes did the same thing. Including the Class-based costumes.
I get what you're saying, but Smash actually does try to do things differently. Hero is treated more like a Generic Class than it really should be, and Monsters are treated like a specific game matters when it really doesn't need to. Smash is strange like that, honestly. Even Pokemon Trainer, who should really be more about the character Red(and later Leaf), was always treated like just another Generic Trainer for some reason. This also happened with lots of Pokemon descriptions too, often describing what their race does, ignoring any very special members of it(Lucario, Jigglypuff, Mewtwo, and Pikachu all being heavily based on the anime in mannerisms and moves, though not every move. Jigglypuff is notable in that it's extremely similar to the anime's abilities while still being a clone of Kirby, somehow working perfectly in conjunction with both concepts). Greninja is the first one to directly be based upon a much more specific member in Ultimate(using the Ash Greninja design in the Final Smash. I think the Kunai was among every Greninja in the anime, so that doesn't really count). But you get the idea. Smash doesn't follow some type of straight character style, no matter what logic should tell you.
