[Nikker]

If I to play some fantasy football here I might go as far as to say the Win8.x Stores might actually be the reason Valve is ditching it. Microsoft could be aiming for a cut of each game sale a la App Store and try tactics to make it difficult for them to run a parallel store on their platform. When you have a business model like Steam running on essentially the same Win8 store the margins get razor thin.

Of course I don't know for sure but for Valve to back out of Windows the way they did it's definitely a strong possibility. Don't forget "embrace,extend,extinguish".

[modeless]

We do know for sure. Gabe has explicitly stated that the Win8 store with its closed ecosystem model is the reason they hate Win8 and are making SteamOS.