Thanks

As Jon said, just the comic book creators’ stylistic choice. Then there’s Spider-Man that has a hyphen!

I agree with the stylism, it serves two purposes: it gives him a name and describes him in one, hes an iron man not just a person Ironman, its emphasis on the fact he is literally in a metal suit.

Interesting question! I'm not going to put this as an "answer", but after some thought it seems to me that it's simply the author's choice. "Iron man" is adjective+noun, just like "super man", who is really "Superman" - although with superheroes there does seem to be a tendency to keep adjectives separate. The Green Lantern, Green Arrow. But I would say it's a stylistic thing. "Batman" is noun+noun, just like "Wonder Woman"... who is always spelt as two words. "Aquaman" is one, "Danger Mouse" is two, "Bananaman" is one ... I think it's probably just stylistic choice.

Iron man = adjective + noun Batman = noun + noun