For various reasons, iOS users are empirically more willing to pay for apps and services. There are more Android users than iOS users, but iOS apps still generate the lion's share of mobile revenue.

Some sources for this (based on a quick Google search, one could probably find better or more conclusive sources with a little legwork):

http://www.imore.com/ios-gamers-spend-times-android-players

http://gigaom.com/apple/ios-users-buy-more-apps-and-pay-more...

http://www.macobserver.com/tmo/article/new_report_shows_ios_...

http://www.theverge.com/2012/3/31/2916556/in-app-purchases-i...

http://functionalaesthetics.eu/why-apps-are-on-ios-first-and...