MS did not endear themselves to mobile phone dev folk.
Back in the day original windows phones ran Win CE 5, then later Windows mobile 6 and 6.5 - these were all code compatible (worst case a recompile).
Later versions of windows phone were not code compatible and a non trivial code rewrite was required - alienating lots of the "early adoptier" devs who wrote apps for first incarnations of WM.
Many of those devs decided against massive rewrites and as essentially it was code again from scratch, changed to competing mobile platforms
.. and then MS wondered why there was a shortage of apps!