Smash is a hard comparison to make to any other fighting game. It's made even funnier when you realize Smash is technically the highest selling fighting game franchise.
Smash uses well known characters from over 30 different series. People grow up with the characters, make memories with the characters, etc. They have personal attachments, so to see them in this huge crossover is really something else. In addition, it's picks like Ryu, Sonic, Snake, Cloud, etc. These are characters that are stellar and can actually fit into Smash.
With a game like Mortal Kombat, it has a cast of characters specific to itself, just like Street Fighter or Tekken. You don't have the same crossover element that Smash does, and the guest characters (imho) don't get the same response simply because it's a different kind of game, frankly. The last time I played Mortal Kombat was 9. It was fun. But X did nothing for me, and this game isn't really doing it for me either. It just seems kinda...odd to leave out characters that have a history with the series or are cool/unique in favor of the guest characters. It's not that surprising, but still disappointing. Rain was cool, I thought at least.