As I mentioned yesterday, I believe that the huge space requirements to install the iOS 8 OTA update are the real reason adoption has slowed.
John Gruber believes the same thing. Gruber, on Daring Fireball today:
This is a serious problem for Apple, because all those 16 GB devices (let alone the 8 GB ones) aren’t going to suddenly gain more free storage space on their own. A lot of these devices might never get updated to iOS 8, but would if the OTA software worked. Unless they can rejigger the OTA software update to require less free space, iOS 8’s adoption rate might lag permanently.