I don't think that people who purchase Android phones want to put out any money for an app. I hear people complaining about $5.00. Well in Apples' App Store it's can cost $10.00 or more.
It's an interesting point. Of course, I've heard some apps making more off ads on Android than pay on Apple. And Google proves the advertising model works.
Although if I'm a developer unsure how my app will be received that pay model is probably better, and maybe why more apps start with Apple first.