A few reasons... 1) Mobile is hot right now and skilled developers are still hard to come by. Employers who need a mobile developer are in a sellers market. 2) High quality apps can be built with a single skill set, if you know your SDK. The good web guys I know need to be experts at multiple languages, architectures, and services. I have one skill that I'm good at, but because no one else has it my career has gotten a nice jump to it compared to my classmates. 3) There is room to be disruptive. The web is 20 years old. iOS development is 4 years old and Android is three. There's a lot less baggage in mobile and a lot more room for a small team of developers to impact the platform. 4) It's cheap to get started with. For under $750 you can put together a serviceable lab to work in.

The bottom line is there are low barriers to entry and a high rate of return.