Xbox head Phil Spencer has explained during a Q&A at Microsoft's Build 2016 conference why Halo 5 likely won't be coming to PC. He also touched on why some games will remain exclusive to specific platforms, stating that Microsoft published games will be released on platforms that make sense, using Ashes of the Singularity on PC as an example.
That's not to say Microsoft plans to only publish titles on a single platform. Forza 6, Quantum Break and Rise of the Tomb Raider are perfect examples of games working on multiple systems. It's more to avoid experiences that won't really work on different platforms (or weren't developed with them in mind).
"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. 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."
As for Halo 5, which is a first-person shooter (we have many on the PC), Spencer explained that the latest instalment was designed from the get-go for consoles. Halo Wars 2 on-the-other-hand is being developed for both PC and Xbox One. While it's not a definitive "no" for Halo 5, but it's not looking likely. That said, Microsoft could have PC Halo plans with regards to 6 and beyond so we'll have to see.