Jesus, any more of this salt over Spawn and I'm gonna develop heart disease.
The reason he's not here: The character's been whored out too much. Todd McFarlane's sold him off so much. He's even been in a Soulcalibur game. Mortal Kombat is better than that. Predator was one of the original inspirations for MK fatalities. Sub-Zero's spine rip? Yeah, Predator wore it better.
Cyrax/Sektor/Even Fulgore? Obvious homages to Predator. This dude helped to define the franchise, it's time he got the tribute he deserved. and I'm gonna come right out and say it: A comic character in MKX would be goofy. Injustice maybe would be okay, but no superheroes in MKX. This isn't coming from ignorance (I was a comic fan before I was a Mortal Kombat fan), but out of a respect for both series, and it just wouldn't fit well.
And so I'll leave you with this: "Yes, we totally need another demon character! This is as good of an idea as adding another palette-swap ninja!"