The SF and MK fans who love both care ^ lol(not saying me, give me MK any day over SF) but since you asked
it's a combination of a few things, the fans who grew up playing both and love both, the fans who are in love with crossovers(some people love them, others hate them) and the fact that crossovers seem to be "the thing" now apparently, from Hulk vs. Chun Li to Yoda vs. Voldo to Subby vs. Batman lol if the idea attracts people enough to buy it, that's all that matters to the people who make them.
P.S. I can't see judging MK9 just yet when nobody has played the game...kind of flawed logic for whoever said that. Just saying, that's kind of like saying well the 2012 Impala will suck because of how it looks...
"Judging a game several months before it's out, much less before playing it is as silly as judging a car by just the looking at it without even driving it" -MKF30
Since we're doing quotes...lol
BTW, honestly no disrespect to anyone in particular but if you are a TRUE gamer an actually, really follow MK and SF then you know damn well MK is NOTHING like SF...MK for one is A. darker, B. has a storyline, C. more developed characters, D. was the first fighter to start the juggle system, E. IS a million TIMES more bloody, violent and F. has totally different gameplay then SF....so please.
The only thing that's the same concerning SF and MK is both are known for being 2D...woah..that's nothing, since we all know there's been 2D games long before SF, Mario, Sonic, Karate Cham, I can go on...so it's kind of dumb to say MK is like SF or vice versa...nothing remotely alike.