Every iOS has been a mess since iOS 7 at least, and the reason seems pretty straight-forward: every year there is an expansion in scope while maintaining a strict 1-year development cycle. This necessarily means that the bugs start to pile up (since the main focus is constantly some set of new features, never fully allowing a true stabilization period), and I suppose its reached a point now where they are evident to even the untrained eye, but the writing was on the wall for quite a bit if you ask me. Just take a look:
1. iOS 7: Major overhaul of the UI means the entire system is going to be in flux (to this day UI doesn't feel as polished as iOS 6, whether you prefer that style or not). Either way, the focus was monopolized by remaking every UI widget. Also, Swift announced.
2. iOS 8: Apple Watch year. Major amount of work getting iOS to run on a third platform (iPhone, iPad, Watch). Surely lots of attention drawn away to get the Watch shipped, plus associated iPhone support for the Watch. This was also the year a new screen size was introduced with iPhone 6 Plus (arguably the downscaling shouldn't have affected mainline iOS stability too much, but just to highlight ever increasing support for different stuff). Also, third party keyboards.
3. iOS 9: Support for the latest gimmick "3D Touch" which required every app to add a bunch of (IMO) useless new undiscoverable affordances as well as long overdue major overhaul of iPad features: multitasking, etc.
4. iOS 10: Entire new class of apps for Messages and associated entire new App Store. Basically a mini-OS in Messages. Also, surely while all this is going on iPhone X is being worked on in the background.
5. iOS 11: Another major UI overhaul with tons of old affordances replaced, replacing Touch ID with Face ID, ANOTHER new screen size added with iPhone X.
iOS 1-3 were great because they did LESS. Its easy to forget that for example Apple used to outsource its mapping to Google. Now Apple needs an entire huge team JUST to have all the data it used to simply get access to. iOS used to also be more elegant because it did LESS -- remember, we couldn't even copy-paste before. These things add up, you can't go from supporting 1 device to 3 classes of devices with at least 4 variations on each and doubling the amount of apps you spit out and maintain without something having to give. And it's not just from a QA side either, the very nature of the phone has gotten more confusing: with every year, we've shoved more and more of the desktop's features into the phone, and as it turns out, there was no magic - if you want to be able to do 100 things, you may very well need 100 new code paths and a 100 overloaded hard-to-learn gestures. Maybe there existed an alternative path to the "dream" of a device as simple to use as iPhone 1 that let you do everything a desktop can, but we certainly won't know anytime soon.
Just think about it, you basically get 6 months of iOS N.0-N.6, then a WWDC where iOS N+1.0 Beta is announced, which of course has to entice people with M "new!" features, and the cycle begins again.