Different form factor / usage devices need different GUIs, sometimes different APIs and OS features
Different form factor, resource & use devices (Phones. tablets, desktop PC/Laptops, TVs, set-boxes, mobile Internet vs broadband vs no Internet) often have widely differing applications.
Applications can only be really GUI independent for the same form factor.
MS have always done this wrong (since Win CE, WWFG and Win9x) and are now making it worse.
They don't seem to get it.