because they saw it as the only way to get developers and apps for the Windows Phone OS. The reason for that is because without a phone presence of sufficient size they'd slowly lose not only the tablet market but also more and more of the desktop. Unlike any other company, Microsoft is 100% reliant on one product, the desktop computer. Sure they make over 30% of their profits from the server but without the server being tied at the hip with the Windows desktop, and visa-versa, they would lose all control of that market and quickly become just another player and lose billions in profits annually.
All this Windows 8 desktop UI change is about forcing, via still owning pre-loading channels, a new API platform on millions so they can market to developers to get on board those APIs and make applications which will also run on the Windows Phone phones and Windows based tablets. It is still all about developers, developers, developers as Monkey Boy once ranted in public.