Unlike many of its competitors who have hundreds of versions of mobile OS on hundreds of different handsets, Apple always boasts of two things, one, that the high majority of its customers use the one and only latest version of its iOS and two, that when a new version of iOS is deployed, most of its customers jump ship almost right away. This has been true for years with both he Mac OS and the iOS. Of course when a new OS boasts significant improvements/upgrades from its predecessor, this adoption rate is even more quicker, like the iOS 7 upgrade (which boasted an complete overhaul from iOS 6). However since iOS 8 is basically a very much polished iOS 7, its adoption rate seems to be slower than that of iOS 7 (via 9to5mac).
See from the graph, that iOS 8 lines up more with iOS 6 rather than iOS 7. The above mentioned reason might not be the only one responsible for this as there might have been other factors, like many of the current iPhone 5 or older iPhone owners who are waiting to upgrade to iPhone 6 (or 6 Plus) and might not see the need to upgrade their older iPhone to iOS 8. Also iOS 8 being the beast it is and requiring almost 6GB of free space and all, might have deterred some of the upgrades as well, at least temporarily. Finally only after installing iOS 8 on my first generation iPad Mini, did I realize how much slower it has become. I am not blaming Apple for it as iOS 8 is a feature packed OS and expecting it to function just as well on my more than two year old iPad Mini like it does on the latest iPhone, is just outright ridiculous. But there might be some of the older iOS device owners who might have finally learnt the lesson than just because Apple's OS upgrades are free and over the air and hence easy to upgrade, you might not merit from the upgrade on your old iOS device. Whatever be the reason for slower than expected adoption of iOS 8 (in the initial few hours of its release!), iOS 8's adoption rate after less than a day of its release is almost equal to Android's latest OS (KitKat's) adoption rate after almost six months of its release (at the time of the figure below).