This was a strategic choise made by Microsoft. They know that WinMo is dropping rapidly in market share, and they absolutely need to keep the MS Office format dominant. That's why they chose Symbian with its great market share, which at the same time isn't a threat for Windows (unlike Maemo/Linux, which can be considered as that, sort of), nor it is owned by their main competitors (unlike Android and Iphone). The remaining alternatives simply have too small market shares to even bother Microsoft.