Almost three weeks since it has launched, iOS 8 adoption currently sits at 47%, identical to the current share of its predecessor, iOS 7. That's a change of only 1% over the course of the last couple of weeks. iOS 7, by contrast, is believed to have hit over 60% adoption by this time last year.
iOS 8 doesn't support iPhone 4, which was supported for iOS 7 and remained on sale in some countries through most of this year. iOS 8 is also significantly larger than iOS 7, requiring more available storage space to download, decompress, stage, and install on-device. People with 8 and 16GB iPhone may have trouble finding that space, and may not be familiar enough with updating via iTunes to go that route.
Although iOS 7 had its fair share of bugs, including frequent resprings (SpringBoard or Home screen windowing system crashes), iOS 8 has had its own troubles, including a delay in HealthKit and an iOS 8.0.1 update that left tens of thousands of iPhone 6 and iPhone 6 plus owners without Touch ID or cellular access for the 24hrs it took Apple to release iOS 8.0.2.
If more people are becoming more cautious about updates in general, it'll be interesting to see at what point the adoption numbers start climbing upwards again.
If you're still on iOS 7 and haven't made the jump to iOS 8, what are you waiting for?