I seriously don't understand people on here. You guys want to know why Microsoft is pushing so hard for Windows 10? Probably because all the features that integrate it with Xbox Live to a lot of stuff like this is already packaged with it, because I know everyone what do nothing but ***** if they had to make another Games for Windows Live to do the same thing.
Not only that with stuff like dx10, which is set to greatly increase performance especially CPU overhead probably make these titles way easier to bring over with the console-like API. Plus, they lost a lot of trust with Windows 8 and 8.1, afree upgrade makes sense that they want to try to gain back your trust with a good operating system.
That being said, I'm hyped as hell for for more killer fighting games on PC, especially with the increased focus on crossplay making the communities even larger. Better step up ArkSys.
EDIT: New information as well for those using this as a means to bash Windows 10, it has been confirmed that yes this is because of the Xbox integration being necessary as "anything you've purchased on the Xbox One version will carry over to the PC version. This also includes cross-buy, meaning buying one version gets you the other" Source: http://www.eventhubs.com/news/2015/jun/16/killer-instinct-announced-pc-during-pc-gamer-conference/
So yes, since nobody wants to deal with GFW type system to be able to connect the two infrastructures, Windows 10 is required for good reason.