They wanted a black founder. Mr. Terrific I guess they thought would be redundant next to Batman, Black Lightning they probably thought would seem racist, Steel is from death and return so he doesn't work. No one else was really prominent enough.
The thing is though, adding Cyborg as a founder was just retarded. The JLA's lineup changed with the times, so they added black people as they were needed. The core 7 in the reboot should have been the team that fought Starro: Clark, Bruce, Diana, Hal, Barry, Arthur, and J'onn. True, there aren't any black people, but that story is from the 1940s. Had they actually taken the opportunity to fix their chronology, we could have seen a real timeline with the fall of the JLA taking place in the 90s (Death and Return, Knightfall, Emerald Twilight) and a more modern group emerge, maybe even with the Titans taking the league's place. And the modern era is where you'd add your black people rather than butchering Cyborg's character history.