Crossovers are nothing new in the world of gaming, whether it's a Halo-themed DLC pack for Gears 5 or the various non-Nintendo characters in Super Smash Bros. Ultimate. However, one players have been binning years for in the world of fighting games is for Mortal Kombat's fighters to cross fists with the roster of Street Fighter. Mortal Kombat's co-creator Ed Boon understands the desire for this crossover as well, but explains on Twitter that making such a game happen is not as easy as players hope.
Ed Boon spoke about why a Mortal Kombat and Street Fighter crossover hasn't happened after another user broached the question. Specifically, the question implies why the crossover didn't come together in both franchises' formative years in the 90s. The NetherRealm Studios head explains the issue comes having two publishers work together cohesively to make such a game happen, which he says is not as simple as "people want it, so do it."
Crossovers are nothing new to either franchise, though, as both have crossed paths with notable franchises inside and outside of gaming.
In the case of Mortal Kombat, 2008's Mortal Kombat vs. DC Universe saw the former's fighters face off with the heroes and villains of DC Comics. It would be the last game in the series released by Midway before the franchise was bought by Warner Bros. Interactive and the studio reformed under WB as NetherRealm. Since then, NetherRealm has jumped back and forth between Mortal Kombat and its DC series Injustice.
As for Street Fighter, players were treated to a crossover with Bandai Namco's flagship fighting series Tekken with the release of 2012's Street Fighter X Tekken. It was meant to be the first of a two-game crossover, with the first game done in the 2-D gameplay style of Street Fighter and the second in the 3-D gameplay style of Tekken. However, Street Fighter X Tekken was met with blowback despite positive reviews after it was found DLC characters were locked on the game disc. Adding to that, Tekken X Street Fighter has reportedly been "on pause" since 2016, though Tekken lead director Katsuhiro Harada insists the game is still happening.
Regarding a Mortal Kombat and Street Fighter crossover, Boon's statement makes sense as it is a problem seen in most media when two separate entities try working together. Typically, both companies will want to have consistent and meaningful input in the project to ensure its franchise is bolstered by the crossover. Take into account how both Capcom and Midway would take little shots at each other's fighting game series during the 90s, and its easy to understand why neither studio may have been in a rush to work together.