Because Mortal Kombat is a hodgepodge of genres and tropes and styles shooting at a dozen directions at once.
In the first game alone, you have your elemental ninja warriors, one of whom is basically Ghost Rider. You have Bruce Lee except he shoots fire and belongs to a mystical order. You have a Terminator guy who leads a crime ring. You have a Special Forces cop lady chasing said Terminator guy. You have a literal Thunder God who for some reason is fighting in a tournament. Van Damne is there except he has a personality and punches people in the nuts. Also there's a green ninja but he's got nothing in common with the other ninjas.
All of these are killing each other so they can eventually fight a claymation-looking four-armed ogre, and after that, they'll fight the tournament's organizer, Lo Pan who also transforms into all the others, and later we find out he's working for a cosmic conqueror who dressed in skull headwear and no pants.
You know how Street Fighter II's cast is basically a bunch of martial arts experts and shonen stereotypes (still tied to martial arts), until you get to the final boss who is just a military guy who becomes the fireball instead of shooting it, who looks like the final boss because he doesn't look like a single other character? Mortal Kombat is basically if every character in Street Fighter was M.Bison. Pretty much none of them belong in the same world.
Meanwhile at DC you got two leads in Superman and Batman, who both belong to radically different worlds and sensibilities and have little in common with pop superheroes like Flash, or space sci-fi Green Lantern, or mythology kitchen sink Wonder Woman, who’s a 180 from the likes of Hitman, who has no business living in the same setting as Morpheus and Death from Sandman, and that's before you get into the Charlton and Fawcett and Planetary and Watchmen and all the other stuff DC's absorbed.
They should complement each other far better than they did.