While admittedly unlikely, depending on how MS pays devs, I could completely see MK11 coming to GamePass at launch. You have to consider...
* That is guaranteed money. Either they get some kind of flat fee or, similar to Kindle Unlimited, an amount for each hour played. And if it is available on GamePass, you can imagine that is a lot of people that will put in a lot of hours.
* It puts the game in the hands of a lot of players at launch, many of which you probably figure wouldn't have bought it yet. This is an excellent way to spread word of mouth and promote the game outside of usual avenues.
* Likely, several of those playing the game on GamePass will pay for DLC. They will have the mindset that "Hey, I didn't have to pay extra for the game anyway..." DLC is basically pure profit margin for devs from what I read.
* I saw a story somewhere that people that play a game on GamePass are actually more likely to buy a game if they like it. They may bank on GamePass members buying the game to permanently add it to their collection.
Still, it is very unlikely, but I could see why they would do it.
I just don't buy it. Mortal Kombat does not need any help getting more casual players on board. I would expect in this specific case that MK would lose sales to GamePass, as I'm fairly certain a lot of buyers only play the game for a week or so to see the fatalities, then stop playing when they go online and get absolutely destroyed.