Primarily because iOS is where the money is, iOS customers tend to be middle-class with significant disposable income, hence they're both the group most likely

Primarily because iOS is where the money is, iOS customers tend to be middle-class with significant disposable income, hence they're both the group most likely to be interested in health related apps and have the income that makes $100 devices fall into the impulse buy category. There's also the secondary issue in that iPhone are much more standardised so it's much easier to test your app on 99% of the phones that your users are likely to be using.