Because of how neatly the good ending of Mortal Kombat 11's DLC ending showed players the potential for a new chapter in the history of Mortal Kombat, and that chapter would be less interesting if familiar faces started showing up for contrived reasons.
[Warning: Spoilers for the end of MK11: Aftermath below]
By the end of Mortal Kombat 11's Aftermath DLC, Liu Kang has traveled back in time to the era of The Great Kung Lao, decades before the original Mortal Kombat game and the events of that tournament. In Mortal Kombat's lore, characters like Shang Tsung, Kitana, Shao Kahn, and Goro were alive at this time (since Edenians and Outworlders have longer lifespans than humans) but many of Mortal Kombat's iconic fighters like Sonya Blade, Kano, Jax, Sub-Zero, and Scorpion had yet to even be born.
Because of this, it would be pretty out of place for these characters to show back up when Mortal Kombat 12 releases. Other Mortal Kombat mainstays like Raiden should not appear either, but for a different reason - Liu Kang is basically Raiden now.
MK12's Liu Kang Will Fill Raiden's Role
Raiden is not necessary in Mortal Kombat 12 because Liu Kang, for all intents and purposes, is the new Raiden of the Mortal Kombat universe. After the events of Mortal Kombat 11: Aftermath's ending, Liu Kang may even be more powerful than Raiden ever was - especially since he is shown to be a God of Fire as well as of Lightning. Liu Kang is the mentor and leader now, and The Great Kung Lao should fill Liu Kang's role as the God's Champion.
While there are plenty of Mortal Kombat fighters throughout the series who are either of indiscriminate or advanced age, the majority of fighters in the Mortal Kombat (2011), Mortal Kombat X, and Mortal Kombat 11 timelines would not be alive while Fire God Liu Kang and The Great Kung Lao are having their own adventure. Many of the fighters introduced in Mortal Kombat X, like Cassie Cage and Jacqui Briggs, are simply too young for it to make any sense for them to appear. Even mystical characters like Sub-Zero and Scorpion were both human before the Mortal Kombat Mythologies and the first Mortal Kombat game, so there's no reason for them to show up either.
Mortal Kombat has a tendency to repeat itself - in stages, in storylines, and in fighters. Some fighting game players grow attached to a certain roster of characters and never want to branch out, and that is understandable, but one of the worst things Mortal Kombat 12 could do for the franchise is just have Sub-Zero fighting Scorpion on Shang Tsung's island again. The Mortal Kombat series has done enough with time travel for the foreseeable future, so anyone who isn't yet alive in the time of The Great Kung Lao should definitely not appear in Mortal Kombat 12.
