If the original Iron Man was in the public domain, would they stop making new Marvel movies? Why, because the movies are currently driving demand for issues of Tales of Suspense #39 or Iron Man #1, and wouldn't be worth it without that?
Many of the classic Disney movies are based on stories that were in the public domain at the time of their original publication.
They keep rehashing old stories because they're lazy or are marketing to fans of the franchise, not because the original is or isn't in the public domain.