Mainly due to storyline reboot (Mortal Kombat (2011)), resurrection as revenants (Mortal Kombat X), and time travel (Mortal Kombat 11).
After the events of Mortal Kombat: Armageddon:
The next game in the series, only known as Mortal Kombat, would begin with the aftermath of Mortal Kombat: Armageddon, revealed that nearly every kombatant met their demise, with the exception of Raiden and Shao Kahn, before shifting into an alternate timeline caused by Raiden sending a message to himself during the time of the first Mortal Kombat tournament, shortly before his own death at the hands of Shao Kahn.
The Mortal Kombat storyline has been rebooted as of Mortal Kombat (2011). Some of the characters that have died in the previous Mortal Kombat games became alive again.
The story retells the events of the first three games, in a new timeline where Raiden receives a message from himself in the prime future after the Battle of Armageddon has happened and Shao Kahn became the supreme leader of all the realms. Because of this plot, characters and scenarios present are the classic ones, bringing the old to new and starting a new conflict.
During the events of Mortal Kombat (2011), several characters died, but in the next installment, Mortal Kombat X many of them reappear, having been resurrected as revenants. e.g. Liu Kang, Kung Lao, and Kitana, among others. There are also characters that already were or became revenants in MK (2011) and were resurrected to their human selves in MK X, e.g. Sub-Zero, Scorpion, and Jax.
In Mortal Kombat X, the souls of the fallen Earthrealm warriors following the Outworld invasion have been claimed by the sorcerer Quan Chi, following a truce between him and the emperor Shao Kahn. These souls have been reincarnated into revenants of the Netherrealm, brainwashed with the sole purpose of serving the fallen Elder God, Shinnok. Only Sub-Zero, Scorpion, and Jax are the known Earthrealm warriors to be revived as humans again.
In the next installment after MK X, Mortal Kombat 11, several characters from the Mortal Kombat (2011) timeline* have been transported through time, decades to the present due to the machinations of Kronika, the Keeper of Time. Some of the characters who are dead in the present reappear as their younger selves. Some of the past versions of characters even meet and fight their present selves.
Kronika: Once again, the Thunder God [Raiden] has upset the balance of history.
But know this - the arc of the universe bends to my will. It is only a
matter of time.
* Just after Kung Lao had defeated Shang Tsung and Quan Chi, as per Raiden's dialogue in MK 11. (Chapter 11 of MK (2011)'s story mode)