Note that some of those people wouldn't really work with the DCEU as we have it, at least not without some serious "Not going to even care about fitting in the slightest". You can't have Wally West without having Barry Allen, for example, and the movie's logic was stretching things enough as is to have Batman old and established. It wouldn't really work to go "Oh, hey, we also had this other speedster guy out and about for a decade, until he died and got replaced by his old sidekick".
Likewise. . . John, J'onn, and Shayera all have an alien origin, and an *independent* alien origin. Introducing them all in the JL movie would be awkward as hell, you'd really need to have introduced at least two of them in earlier movies ( their own or other people's ).