These characters just aren't big enough names to the general audiance to be the huge boxoffice hits that WB would expect them to be. At least not until after appering side by side with the moviewise well established Superman and Batman. And if they'd finally manage to get another heroes besides the Big Two going, but it wouldn't be THAT successful, i'd seriously doubt they'd try more of the "lesser known" DC-characters.
Besids, trying to build up a Justice League movie like that would be like constructing house of charts, that could collapse back upon itself at any time on the way. Also i'm more then just sure that they woulnd't come that far with the plan, let's just assume they'd be able to do one respectively two more for the Batman and Superman franchise, then follow up with a (in my mind absolutly unnecessary) team up flick with both characters and along the way kick off a Flash, Green Lantern and Wonder Woman Solo-Movies, so the JLA is finally in reach. Do you really think they'd be able to get all those actors from the solo-movies, that would be big stars now, together for a team-up flick where they would've have to share the spotlight? If not, then what? Recast so that fansboys would be *****ing again? Or just screw the movie that they would've build over like ten years? But even if they'd be able to get them all togher, it would just cost them WAY TO MUCH money. Even with unknowns, a film like that would be expensive enough, so how much would the movie have to make at the box office in order to be profitable/successful? 800 millions? Not to mention that if the director of the League would have to use the actors and the interpretations of the characters that were casted/created to fit for solo movies, they wouldn't necessarily complement one another and make a balanced overall picture that'd be required for a good team-up flick.
So seriously, it might be a nice little fanboy pipe dream to get a Justice League movie like that, but if you think about it rationally, that just woud never work out in reality.