They want to avoid fragmentation. It's okay to have all these different ipads with confusing names like iPad Mini Air Pro (jk) as long as they are running one version of iOS for Apple to keep updated.
I'm kind of mocking them for this but at the same time I recognize the sense it makes, especially compared to the situation with Android.
Fragmentation will always exist even with apple, cause apple wants thier iTunes revenue to keep rolling in from older devices that cannot run the latest OS. I just don't need apple telling me that I have to update the OS if I'm happy with the one I'm on. iOS 10 really does nothing for me........ to be honest iOS 9 and 10 are so damn similar. Only thing I dikslike is the battery on 10, on iOS 9 my iPad Pro has amazing battery, my iPhone 6s Plus, which just got replaced, awful have an S8 + coming tomorrow, if that has great battery life, it's a deal breaker.