The WB conundrum has been discussed ad nauseam, you really have to take a look at their cinematic history.
Wall of text below so I am spoilering it because NTers are afraid of them. Warning: Spoiler! (Click to show)
They're old CEO didn't like risks and when Schumachers Bat films were lambasted by the critics and flopped, they become discourage and more cautious. Hell it took them 19 years before they were brave enough to release a follow up to a Superman film, that's how bad Superman IV was and how cautious they are. They tried to release the newly documented Superman Lives and that one didn't get made because of the string of box office bombs by WB (it's a good watch for comic book fans). Then when they were going to do a JL film by George Miller (yes, Mad Max director) the film got shut down because of the writers strike (they were already close to production and all cast were on hand to shoot within a week or so iirc) and in the end the studio probably felt that it was again more risk and pulled the plug.
It also didn't help that the budget for Superman Lives (and I believe the failed JL film) was added to the Superman Returns budget so it got inflated and the film looked more of a flop that it actually was, not saying the film was great or anything at all but it made close to those XMen films and back then, it wouldn't have looked as bad if the budget were lower.
Then there's the GL flop that didn't help at all and all other films got shuffled around in limbo and went no where and that is despite Nolans Batverse success.
But back to the CEO, once WB got a new one last year (maybe two years ago, I forget) then look what happened, a DCEU announced a full movie line-up til 2020 including the much criticized Cyborg solo film (this is how much risk they are willing to put out there now). Then all these tv shows got greenlit for any channels wiling to make them (Flash, Arrow, Gotham, Supergirl, Constantine, Teen Ttitans, Legends of Tomorrow, etc...). Those are no coincidence that it happened all of a sudden once the new CEO took over.
In the end, it just wasn't as easy as simply putting out any characters out there on the big screen, for a long while WB/DC was fine with one film at a time. Marvel Studios revolutionized how we view these films and the whole cinematic universe and we have been spoiled. It's a bit unfair to simply say "WB should have done this" when what Marvel did was unprecedented. IM was a huge risk for Marvel too and had that failed there would be no MCU today, keep in mind that before that they witness Hulk, Ghost Rider, Punisher, Daredevil and Elektra fail miserably. Not by Marvel Studios but you get the idea. Outside of the bigger names (XMen and Spidey and Batman) most of the tier 2 and tier 3 heroes weren't a big success so someone had to take that jump and Marvel Studios did and succeeded. It didn't help that the movies weren't that good but again it just wasn't that easy to make comics a success.
And all that is really just a small piece of a huge pie when it comes to comic book films. I am surely forgetting and missing plenty of details there.
Again I suggest watching the Superman Lives Doc, you can see how comic book films were created and viewed back then, they snuffed on comic book writers because they felt their writing wouldn't translate well to the film.