It's very simple really. Microsoft got into the game, too late and too ignorant. They are only starting to change now and when I say change, I mean SLOWLY change the way they look at things. Just look at their Lumia phones, almost EVERY ONE of them after the Lumia 920 had had some caveats or problems. For example, my Lumia 1520 has the touch/swipe issue and nothing can be done about it sadly (even with the screen pressing). A big flaw for a premium device. You simply don't see this on Android phones (even the behemoth Note 3 doesn't experience this problem).
Lack of apps and lack of novelties are also pulling WP behind. I like Windows Phone with its simplicity and ease of access (Live Tiles being the main attraction). However, the software just doesn't live up to the simplicity it offers. Like a lot of the other users on this forum have said, Microsoft needs a game changer and from what I can see, usability is the way to go. Can you imagine having a phone that will automatically update your settings by understanding your preference?