As noted in the Season 1 episode, a foreword by the Director notes that the series has moved beyond the core game story line(s).
The storyline with Liu Kang diverges from the story previously seen in the Mortal Kombat games and movie adaptations of the games, in that Liu Kang is not the protagonist or main Earthrealm Champion. In fact, MK2 (the game) states that Liu Kang became the Earthrealm Champion for the Shaolin monks because Kang Lao declined to fight. In Legacy, this has been flipped around, with Kang Lao becoming the Shaolin monk to fight for Earthrealm.
In the MK storylines, many Earth fighters are tempted and fall for the Outworld recruitment tactics. This time, Liu Kang falls.