For starters, many simply can't upgrade - iPhone 4 users for instance - as the new OS simply isn't supported on devices that old. There's also likely a healthy contingent of Apple users who are happy to stick with iOS 7, either out of preference, or for performance reasons.
iOS 8 has been reported by many as causing a noticeable slow-down on some models, particuarly the iPhone 4S. Alongside these groups, there'll also be many who simply just haven't got around to upgrading yet, or simply don't know how to. The figures will have been especially bolstered after Apple managed to sell upwards of 10 million iPhone 6 and iPhone 6 Plus units over the launch weekend, all of which will come with iOS 8 installed as standard.
Interestingly, there's also a remaining 5% of Apple device users running operating systems before iOS 7. That's old school. The new statistics show up on Apple's developer support page for the App Store. It's handy for devs to know how many people are using a given OS, as incentive to provide app compatibility for olders systems. By comparison, just 24.5% of Android users are using Google's latest 4.4 KitKat operating system.
