Haven't gotten far, but all three platforms have the same "stuff".
Ways to draw the app on a computer and ways to write code to make stuff work. App Inventor, Visual Studio, and XCode, all have a graphical interface that allows the user to draw the user interface and then you write code to make things work.
If you want to do stuff that's already done on the web — you HTML5 and JavaScript. Data is stored on the web and you use API's to get the data.
Most of your mobile apps are just interfaces into a web app or server. Even the games.
Here's the rub and why people aren't developing for the Windows platform. When I worked on the mobile app most people used Macs. That's because they can develop for both Mac and Java at the same time. Most people who develop on the Windows platform don't get Unix (actually I do).
And this is putting a bunch of stuff together — I've been to a ton of workshops by Microsoft where we developed for the Windows Phone. I've been to a IBM workshop where they showed us how to develop on Unix using their databases. That's my first Android experience. And I've started just playing with Apple.
The Windows Phone is the easiest user interface to master, it looks like Apple is second and then Android. However, a developer can develop Android on any machine. To do Windows Phone or iPhone, they are limited to an OS and hardware.
So why buy another set of developer machines for something that only has a limited market share?
And that's the answer.