It looks like Call of Duty: Infinite Warfare is coming to the Windows Store. It's not searchable on the Store at the moment, but the page for Modern Warfare Remastered (opens in new tab) is discoverable, and it opens the same page in the Store app, where it's marked as not available for separate purchase with links to two versions of Infinite Warfare that include it, the 'Digital Legacy' and 'Digital Deluxe' editions.
You might want to think twice before you make that purchase, though: Activision's "Call of Duty: Infinite Warfare and Call of Duty: Modern Warfare Remastered for Windows 10 on Windows Store" FAQ states that Microsoft's edition of the game cannot be played online with versions that were purchased elsewhere.
Question: "Can I play Call of Duty: Infinite Warfare or Call of Duty: Modern Warfare Remastered on Windows 10 on Windows Store with my friends that are playing on another PC platform?"
Answer: "No, you can only play these titles with other users of Windows 10 on Windows Store."
It's also not a "Play Anywhere" release, so buying it for the PC on the Windows Store will not get you access to the Xbox One version, and vice versa.
That's pretty straightforward, isn't it? If you're buying it for the multiplayer—and most people probably are—then buying it from the Windows Store puts a real crimp in the number of people you can play with. (You could argue that the Steam version suffers similarly for not being compatible with the Windows Store release, I suppose, but I'm willing to bet that the Steam playerbase will be a whole hell of lot bigger than Microsoft's.)
My guess is that the Windows Store version, which is Windows 10-only, is for various technical reasons not compatible with the Windows 7-supporting version that's on Steam. But it's a bizarre situation, regardless of the specific reasons behind it. If they can't be made to play together, then why bother bringing the game to the Windows Store platform at all? Ultimately, all it does is shine a bright light (again) on how it falls short.
I've reached out to Activision for more information, and will update if and when I receive a reply.