Because to put an app in the iOS app store requires you to pay a subscription fee. Most developers might see it right in that case to price the app. But in Kenya, no one buys apps. And appstore doesn't accept mpesa.
On the other hand, in google playstore, its free to put up your app. And hence most downloads are free.
Plus most Kenyans own Android phones. So it's not worth it to hire an expensive iOS dev to make an app.