If you look at it that way, you have to ask why characters like Gambit etc, or more classic characters like Beast and Angel werent in the first X-Men film.
When translating a comic book to film, what came before, and what fans expect, are the two things movie makers consider least.
Granted Batman and Supes in the movie would probably draw more, but if it meant them not doing or doing sequels to solo movies late, they would more than likely go with dropping them. The rationale being that they would make more money that way, even if JLA didnt draw as much as a JLA with them in it!
