There have been a lot of Xbox One games that have been coming to Windows 10, including the just-released Killer Instinct, Quantum Break, and Halo Wars 2. However, the one Xbox One title that everyone is curious about is Halo 5, and Microsoft's Phil Spencer has stated that gamers shouldn't hold their breath waiting for it.
Spencer explained in anthat not every Xbox One game would be getting a Windows 10 counterpart. "I don’t want to make it some kind of artificial mandate, because then I think we end up with ‘Frankengames’, games that really weren't meant for a certain platform. And because some suit said, ‘Hey, everything’s gotta run on both platforms’, you end up with something people don’t want," he noted, adding "You should expect it when franchises look like they belong on both platforms, but I wouldn’t go so far as to say it’s a mandate for the studios because it’s not."
However, that doesn't explain why Halo 5 wouldn't be a good fit on PC. The reasoning Spencer gave for Halo 5 not coming to PC was confusing, at best. "In terms of Halo FPS on PC, I think there’s a ton of opportunity for us right now, but I don’t want to get into a world where we’re looking back, like at Halo 5. It doesn’t mean there’s nothing there that could ever end up on PC, but I’d much rather look forward with what our plans are," was his response.
The real reason may lie in the fact that Halo 5 is the tentpole game for the Xbox One. While other console "exclusives" like Quantum Break being on PC won't affect Xbox One sales, having Halo 5 available on PC at this point would be probably be very detrimental. It took two years after the Xbox launch for the PC version of Halo to be released, and three years for Halo 2; no other Halo games has been released for PC since. Additionally, if Microsoft ever had plans for Halo 5 to get a PC release, they'd probably have to port the The Master Chief Collection, which includes all Halos save Halo Reach released before Halo 5.
There is also the matter of multiplayer. Microsoft's big push has been Xbox One/Windows 10 crossplatform play, and in the past, gamepad users have had a hard time competing against keyboard/mouse users when it comes to first person shooters. Not including a crossplay feature on Halo 5 would be unseemly at this point.