Why does The Rocketeer work? Wait, does The Rocketeer work? I've only seen it two-three times; and I see that it barely made back its investment in domestic box office (and hardly did more internationally). But I think if you released it now, people would be a lot more interested in the Art Deco-design. And maybe Disney might market it under a slightly different label so people don't think it's a kid film.
But no matter how poorly it did, it's a fun, light-hearted movie, even though it involves Nazis and gangsters and men down on their luck. How did it do that? Part of the issue is that no one is just waiting around for the protagonist to fix everything. Part of what makes it work is that the main characters have multiple, conflicting desires--even if those are pretty ordinary desires (usually career success/romantic success--and who can't identify with that?).
And as wacky as it is--Nazis! gangsters! Howard Hughes!--it keeps pretty close and simple at first and only builds to that wackiness. It's a very easy film to watch, since the world is totally normal at first and only slowly spins away from what we know.