With only eight films currently released, there are naturally still many missing DC Comics characters in the DCEU. The company dates back to 1934, so that's a lot of ground for the films to cover. Even if the comic book universe has already united the biggest heroes in Justice League, there are still plenty of powerhouse characters that have been left out of DC cinematic output.
For the characters that did make it in, DC has decided to stick with a gritty aesthetic in regards to its films. That grim and grounded style of Zack Snyder and Christopher Nolan works with some characters, while others have been clearly underserved. The company's cinematic failures before the DCEU seem to be affecting their decisions, as well, as neither Robin nor Hal Jordan has gotten a fair shot onscreen since their failures in Batman and Robin and Green Lantern, respectively. Regardless of the reasoning, the DCEU has done a number of characters dirty and needs to make it up to them.