It did everything right
'The Rocketeer' may have flopped/underperformed at the box-office but more than almost any other comic-book movie until 'The Avengers' it really captured the tone and look of the characters it was adapting.
It's a fun, upbeat, exciting, good-looking movie that is perfectly pitched for the family but never patronises its audience (the films it most emulates are the 'Indiana Jones' movies, albeit less bloodily violent). The filmmakers went for a unknown lead who actually looks like a square-jawed comic-book hunk (an approach which paid dividends with the Christopher Reeve 'Superman' movies), rather than picking a star or someone perversely 'quirky'. The leading lady is a traditional classic beauty, who can also act. The villain is suitably nefarious, but credibly dashing and thus believable as someone who would be able to fool the world. And the action is thrillingly conceived and staged without any of that OTT editing that now mars many action movies.
It's a shame the film didn't become a box-office hit. I recall enjoying it thoroughly at the cinema as a kid. However, on the bright side at least it was never overshadowed by any almost-inevitably inferior sequels.