Ever since the revived Killer Instinct launched in 2013, you've had to download it through one of Microsoft's online shops, whether it's on your Xbox One or your Windows 10 PC. However, you won't have to go that route in the next few months. Iron Galaxy has confirmed that KI will be available through Steam sometime later in 2017. More details are forthcoming, so it's not clear if there will be anything new or any incentives for existing owners to make the leap. The very fact that it's happening at all is important, though, and it hints at an emerging trend.
Source: Killer Instinct (Twitter)