Mortal Kombat Guest characters make no sense.

If you truly think about it...
Kratos, Joker, Freddy Krueger, Jason Voorhees, Leatherface, Spawn, Alien, Predator, Robocop, Terminator... You got horror movie slashers, Horror Cyber punk Antagonist turned hero, Sci-fi Cyber Jesus policeman, a Greek demigod who beat up the Greek oantheon and Viking gods, a Demon general who rebelled against the devil, and The Joker... They don't make sense at all...

So, the only "link" is excessive violence.  Other than that they make no sense... and I mean Norimaro levels of WTF this makes no sense!

Since Mortal Kombat isn't seen that well in the East, they can't get some popular fighting game characters to cross over. Capcom's Darkstalkers, Devil May Cry, or Resident Evil casts could Totally work in MK...
Tecmo's Ninja Gaiden could work...
Rare's Killer Instinct characters could work as guests...
But let's be real, Mortal Kombat blew their load on their limited selection of guest characters. They can't do Chucky due to size. Pinhead is not a fighter...

Maybe Pennywise (hopefully the Tim Curry version)
Ash Williams would be like the biggest two that they could get.

But seriously, ROBOCOP!? WHY, MORTAL KOMBAT, WHY?
So he can fight Carl, the drape guy?
Sure, Sheeva and Sindel are nice, but Freaking Robocop? It makes no freaking sense!!