What
sinnerjohn said but also the business models of Apple Vs Google are quite different.
Apple make quality hardware and software that you pay for. Most/many developers produce quality software because they can earn a living doing so. iPhones and iOS are apple products. Apple monetizes the app store by taking a cut of each developers/app's income.
The Android OS is not Google's product. Android phones are not Google's product. Google's product is advertising, it monetizes the play store by inserting it's advertising infrastructure into the ecosystem. Google incentivises developers to make free software by providing a way for them to make a living using advertisng within thier apps. Because there is such a large 'skewed poor' userbase compared with iPhone users that use low end hardware, developers are less likely to make a living from selling software (and remember the variety of hardware making development harder as
sinnerjohn pointed out) and more likely to buy into Googles advertising scheme.