What's the big deal with it being on W10 store and not steam anyways? I'm not familiar with PC platforms yet but logically the only difference would be you launch through the W10 store and not steam?
Because W10 store apps are Universal Windows Apps instead of Win32 apps, they impose a lot of limitations to the users an take control away from him.
It's like iOS App Store.
To add to this, UWA means not having:
* Performance analysis overlays.
* Graphics injectors.
* Integration of useful external applications like Mumble.
* Community fixes or feature extensions.
* Freesync.
* In-depth modding.
* Support for popular peripherals like the Steam controller. (!)
* Some GPU features such as SLI or adapter switching.
* Actual exclusive full screen mode.
UWA runs contrary to a lot of things that have driven growth and innovation within PC gaming, and for many the reason they choose PC gaming over others. UWA essentially reduces the ability to use external applications of your choosing or modifications, injections etc. Want to use OBS to twitch Stream or stream to a different service or record with Shadowplay? No you can't unless MS deem it appropriate and officially support it. You are restricted to their solutions and choices alone, rather than the freedom PC games have had for decades.