They had no developers familiar with these Microsoft APIs as Windows Phone was starting from scratch with new APIs that were incompatible with neither Win32 nor WinCE/Windows Mobile. The only thing that could save them that they were betting on was that these new APIs were also made available on desktops, but were pretty much ignored by developers there as well and ended up largely replaced with an even newer set of technologies.

Compare that with Maemo, where both GTK+ and Qt were first class citizens and which had an army of developers familiar with Unix and X11 before Maemo even existed.