Iphones come in a standard size, android developers need to plan on all the different screen sizes, processors, etc.

There's like one iphone a year, new android devices come out all the time, that can drive a developer crazy.

Android users have way more free apps to choose from, which makes it harder for them to actually cough up money to buy something.

Pretty much most of the iphone users are on the same OS, android keeps updating, but not all their users update