Zack Snyder's Justice League introduces heroes and villains alike to the DCEU, including an original member of the Justice League comics, the Martian Manhunter. The shape-shifting extra-terrestrial from Mars, J'onn J'onzz, finally reveals himself in Snyder's Justice League after masquerading for several years as General Calvin Swanwick (Harry Lennix), a character who has reoccurred in the DCEU since Man of Steel. This reveal, while an intriguing moment, creates some questions about the character's past.
In some comic iterations, Martian Manhunter came to Earth after his own race was obliterated and all life wiped off the planet. It's a sad story, and also one of the key motivations the Manhunter has for protecting Earth. He is determined to ensure that such a tragedy doesn't repeat itself. While we don't know the backstory for Snyder's Manhunter just yet, it's still curious for Manhunter to stay out of Superman's previous battles in Man of Steel and Batman v Superman. J'onn J'onzz is hiding as General Swanwick, a high-ranking military official with enough status and clearance to deal in both national, global, and even extra-terrestrial affairs, which gives him some ability to protect the world, but not as much as joining Superman directly in those past battles could have.
Although, as Swanwick, he strategizes within the constraints of his rank against General Zod, he also allows himself to remain as disadvantaged as all the other humans whenever the fighting starts. In the comics, The Martian Manhunter is one of the mightiest beings that DC has ever created, possessing more than enough power to fight alongside Superman. In Man of Steel, when Zod starts decimating Earth with the World Machine, engineered to terraform planets until they resemble Krypton, the Martian Manhunter must have been having flashbacks to Mars' annihilation. Even though he uses General Swanwick's authority to convince the United States military to work with Superman, his role in the defense of Earth is minimal. Zod is endangering the entire planet, not just a city or a country, so if there was ever a time when the Martian Manhunter needed to act, it was during Man of Steel.
Similarly, when Doomsday is born and starts tearing apart Metropolis in Batman vs. Superman: Dawn of Justice, J'onzz didn't ditch the Swanwick disguise and go fight alongside Wonder Woman, Batman, and Superman. Although Swanwick tries to stop the U.S. military from nuking Superman and Doomsday, the President overrides him, so at that point, after Doomsday goes atomic, there's really nothing that should be holding the Martian Manhunter back. If he blew his cover, he might have been able to intervene before Superman perished.
All that being said, the Martian Manhunter is in an optimal position for covertly watching over both America, and the rest of the world, as General Swanwick. He may have perceived the threats of Zod, Doomsday, and even Steppenwolf, as ultimately manageable for Earth's other heroes, and if so, he was ultimately correct. Even so, his decision to stay out of past threats is something that needs more clarification (which may never come as Warner Bros. says they have no intention to make any sequels). Also, the peril that Darkseid places Earth in is exponentially worse than what came before, which justifies why J'onn J'onzz came to Bruce Wayne at the end of Zack Snyder's Justice League, after Darkseid discovered the Anti-Life Equation resided on Earth. Still, that's cutting it pretty close.