Mostly becuase I think they keep it closed to keep their hands on the cash from apps, not to 'dictate the user experience'. What if I'm old enough to cross the street by myself now?
I must be more cynical.
Maybe you are lol. If you look, they make nothing (compared to their hardware business) from the App Store. The App Store, from the start, was aimed to be a neutral business.
You need to remember, continual iOS develop costs money that is probably fueled by whatever extra they make.