Because of something on Super Friends?
Like I've said time and again, I was introduced to Cyborg before Martian Manhunter because of
Justice League. I prefer Cyborg over Martian Manhunter because of watching and learning about the characters from reruns of Super Friends before the JLA show was made.
When JLA the animated series was made, I learned about MM and still didn't care much for him, and I do feel he can be replaced as a founding member. I think he'd be good for a sequel yeah, but I don't quite see the need for MM in the first movie.
And like I've said countless times, Cyborg doesn't have to stay in the League at the end of the movie, he's be used to open up the door for the Teen Titans movie, the same way I expect the JLA movie to lead to a Wonder Woman movie, the Batman reboot, the
Man of Steel sequel, as well as a possible Green Lantern sequel, an Aquaman solo movie, and maybe a Flash solo movie.
Along with:
Firestorm
Black Vulcan/Lightning
(Both of whom do deserve at least a nod in the JL franchise)
Apache Chief
Samurai
El Dorado
The Wonder Twins
Gleek
Yes, historically he was part of a group of Justice League like "Superfriends" 13-14 strong. He's not one of the big seven, and not deserving of inclusion in their ranks.
His inclusion in this obviously "diverse" supergroup is hardly a good reason to use him in a serious JUSTICE LEAGUE film. While I think a lot of the general audience does get the "Superfriends" reference when its made, THE SUPERFRIENDS/SUPER POWERS is pretty dated and has been largely forgotten among nonfans. And its hardly comic book mythology canon.
While all that may be true, unlike the above diverse superheroes, Cyborg has built a strong fan-following and earned his recognition in the DC universe; from the Teen Titans comics/cartoon to the New 52 JL inclusion. It's not so out of the blue to prefer him over Martian Manhunter.