Why not webOS (and Android) ?
Well its because Android is doing so well that even Microsoft is contemplating on selling Windows Phone with Android-support.
Would you buy a Microsoft Lumia device that can sideload Android Apps?
And before you say it is false, nein, it is true. It is in Microsoft's Research Lab.
Its just not realized yet, and may not be. However, MS themselves honored the curiosity.
It seems like they're desperate for a piece of the mobile market, so they should've either acted sooner, or offered better products, or just become a service.
I mean imagine an AOSP that's stripped bare. Add in Metro for UI, Xbox Live for games, movies and music, Windows Media Centre/Player for playback, MS Office for office, Cortana for personal assistance, HERE for Navigation, OneDrive for cloud storage, Outlook for email, Skype for messaging, Bing for Search.
vs (Google's) Material Design UI, Play Store for content, generic google media player, Google Docs for office, Google Now for personal assistance, Maps for Navigation, Google Drive for cloud storage, Gmail for email, Hangouts for messaging, Google for search.
Also ecosystems for Apple, Amazon/Yahoo, and open-source ecosystem.