Me too. The creator himself even is cool with it so hopefully this will happen finally
Ed Boon at the time for Spawn inclusion was stuck in a situation on whether to include him in MKX or Injustice II from what he mentioned via Twitter back then. Ed Boon acquired the rights for Spawn ages ago from what he implied but just never used it because he couldn't decide on which game to include him on. There might be some reasons as to why they waited now, and possibly had potential issues with reasons in the past on why Boon couldn't just decide during those times for MKX/Injustice II.
So for Injustice II, this is entirely a DC game, thus including Spawn would be dealing with complicated 4th barrier entries and more meetups between companies and contracts. Basically letting DC know that they want to include Image Comics (Publishers for Spawn) into a DC universe type game. The issue is that Injustice II is not Smash Bros. The game isn't that grand of an opening to just bring anyone in from other comics. DC has some leeaway to it but the Turtles pretty much sealed that deal along with Dark Horse Comic's Hellboy without creating too many problems for DC to where DC might start questioning Ed Boon on it. Todd McFarlane (creator of Spawn) doesn't care which game he wants Spawn in, but for a game like Injustice is a bit on the topic to how DC feels about it because it's their game (technically speaking). Granted that DC has no problems with Spawn, but it might be the matter of avoiding complications between the two (on top of already adding Hellboy and TMNT).
For MKX/XL, it might of been a time where they already had all content ideas planned out and may have acquired the rights for Spawn right after it, who knows. Ed Boon was pretty stuck on this.
Personally? I think Spawn fits on Mortal Kombat the most due to how edgy, violent, and dark his comics were. Not a slight bit on lightheartedness for Spawn. Some say he also fits in DC which I don't disagree with either, but he just looks like he fits Mortal Kombat entirely in opinion of mine.