WP8+ has, by far, the broadest support for programming languages. You write programs in C#, VB, C++, JavaScript, etc. And Visual Studio is *far* superior to the iOS and Android development environments. The Win8 requirement isn't a obstacle for anyone but the most casual developer.
IMO, the app gap on WP8+ is due to:
1. Lack of mindshare in the public
2. Lack of marketshare
3. No easy way to generate a WP8 app from iOS or Android source
4. For games, lack of OpenGL ES in WP8
Items #3-4 are more nitpicking; items 1-2 are the biggies. No one cares about WP other than the people on this website. I don't know how MSFT can fix that public perception problem. From the ISV perspective, I think it would be worthwhile for MSFT to create a Xamarin-like version of Visual Studio that can generate apps for all three platforms (get as close as possible to creating an app for iOS, Android, and WP8+ with click of a button).