It's called Hollywood being stupid. Morons haphazardly read the comics and/or game lore and then poop out their own version of the story. Just look at Scorpion and Sub-Zero being all buddy-buddy because the dumb ass writer (Kevin Droney) decided they'd be "slaves" under Shang Tsung's power.
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It's no fucking different in 2019 now is it? WTF is Sub-Zero doing working for the man he was supposed to be hired to assassinate? Kabal on Outworld's team? A black Mileena?
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Nothing is going to do the games justice. Ever. Just consider it another one of Kronika's or Firegod Liu Kang's many fucked up alternate realities within the multiverse cluster fuck engineered by NRS. Ignore the lore you fell in love with and just try to enjoy the shit Hollywood squeezes out of their hairy, cum-filled buttholes as best you can. It's better than nothing and it will certainly always be better than the Tekken, Street Fighter, and Dead or Alive films.

Wasn't the original storyline in the first arcade game centered around Shang Tsung inviting fighters to his island?

In the movie, Sonya was lured onto the boat/tournament through Kano by Shang Tsung. So she wasn't invited by a scroll as they were in the original arcade game. It was also movie lore Shang invited Sonya to be his own sex slave and then kill her in the tournament as an easy win.

In the movie, Johnny was invited by Shang Tsung disguised as Master Boyd.

But yes, the whole Scorpion/Sub-Zero friendship was added to the movie.

Remember - Mortal Kombat arcade game was literally a ripoff of Enter the Dragon. Bloodsport used the same basic plot - "the best fighters all over the world compete on a tournament"

Sex slave? Get your head out of the gutter. He wanted an easy win for Outworld. That's what he meant when he said he has special plans. It's not because he wanted to finger her poop hole, okay? The question really is, why Sonya of all people on Earth? Because that's how nonsensical Hollywood's writing is. At least in 2019's film, they kind of acknowledge that Sonya was there by happenstance, not because she was an actual invited and/or chosen warrior. Like in the original comics, I guess she earned her spot.