Ed Boon talked regarding his desired DLC guest character. DLC characters are a tradition in the Mortal Kombat games.
For a long time, the fighting game genre was dominated by Street Fighter. And as a result of this dominance, Ryu, the face of the series, has become a cult character. Boone really wants this character to appear in Mortal Kombat.
Apparently, the delay in this matter may be due to Capcom. But fighting games can hardly be called direct rivals. The problem is most likely that Mortal Kombat has a more mature audience and Capcom may not want Ryu to receive and perform Fatalities. However, these are just guesses.