It all comes down to the user base of Windows Phone. Plain and simple. Asides from a few stupid things like Apple's stubbornness on bringing iTunes to Android, companies aren't withholding apps simply because their big jerkface meanies. Windows Phone's market share has never really risen much higher than 3% of the market and it's declining. Without the user base, app developers just aren't going to want to support it unless a platform does what Amazon and Blackberry did by making their phones Android compatible. And without a compelling app selection, users aren't going to buy Windows Phone over Android and iOS. And it doesn't help that Microsoft made a really ****** mobile OS to begin with. It's a big reason why Microsoft has become platform agnostic with their services on iOS and Android as opposed to having Office, Skype, Outlook, Minecraft, etc. being Windows Phone exclusive selling points.
Think of it as the same logic as to why developers and publishers don't want to support Nintendo platforms. Asides from the dickish EA, the other developers/publishers gave the Wii and Wii U a real shot, but their efforts were burned by the small Wii U user base on top of that user base having no interest in games that aren't developed by Nintendo.