The reality is that as the last few decades have gone on, Cyborg has become more popular (both with writers and fans) while Martian Manhunter has waned. I like Martian Manhunter a lot, I think his whole “I understand I’m not part of this world, but love it’s people” thing is something that actually very few other superheroes have that angle on. He’s not 4th wall breaking but he is an objective observer on issues that play out in the DC universe that are influenced by our own society in real life.
But Cyborg has a gun for a hand and shit, so pew pew.
Fake Edit: Seriously though, martian manhunter is arguably stronger than superman, factoring in his ability to shapeshift, to phase and to control/read minds. If you think writing superman is hard because basically he can’t be beaten, try doing it for someone with even more tools in his toolbox.