Mortal Kombat is not a series centered around non-video game characters, while Kingdom Hearts is. It's a series where you travel to various worlds based on Disney movies, and it's hard to represent the series without Disney characters.
And where like 90+% of the main characters are KH OCs anyway, with it revolving mainly around those OCs rather than Mickey, Donald, and Dingo, who are just here to be marketable, have some cool scenes every once in a while, and there was also some times where Mickey actualy advanced the plot, like 3 times. While the KH OCs did it every single other times. And there's also many of those OCs, between the main trio of Sora, Riku, and Kairi, Ansem, Ansem the Wise, Organisation XIII (as their names indicates, they're 13 members of an organisation) + Namine and Xion, Ventus, Vanitas, Aqua, Terra, Eraqus, Xehanort, Young Xehanort, 3 whole different sets of mobs, many more from the 2 mobile games, and Yozora. KH actualy revolves around the original characters, unlike what you believe. I would know, I'm a fan of the series.
Also, "hard to represent the series without Disney characters" is a non-argument because the KH OCs are owned by DIsney, which means that they are Disney characters (yes, the time travelling silver haired pretty anime boy who is
depressed over the death of his friends is a Disney character). But even then, why not take a look at the FF representation in Smash 4? You will be surprised.