Isn't it obvious? Money! The WP marketshare is pitiful compared to iOS or Android. Why develop something that could get seen by a hundred people, when you could get it seen by a thousand people? Its the classic catch-22. WP doesn't get more developers until its marketshare improves. The marketshare doesn't improve until we get more developers making apps. Or so they say. In actuality, apps don't sell an (mobile) OS. Everything that makes it unique sells it. If its not popular because its popular, it needs a "gimmick". For example, the GS4 sold well because Samsung is popular (for being popular), and all those gimmicky features were "cool".
WP could have a much better market-share if OEM (and MS for that matter) would truly differentiate WP from Android and iOS. The 41mp PureView camera is cool, but its a niche product.
Anyway, I'm getting off-topic. Simply put, WP needs a bigger marketshare before more developers will consider developing for it.