Apple iOS updates are usually snapped up, much more so that Android updates. Android has a more fragmented environment, and new versions aren't available to all systems at once (and sometimes never). Apple's ecosystem makes more sense in this regard; and Apple device is an Apple device, there are not multiple brands and a wide range of hardware, and so updated versions of iOS tend to be propagated quickly. iOS 8 has been noticeably slower than usual, though - currently iOS 8 adoption is less than 50%. Maybe it's something about the number 8? Windows 8, I'm looking at you.
More practically, there are likely two main reasons - the initial buggy release may have given some people pause, and they are waiting until things are sorted out - maybe till version 8.1. Apple mobile device users have experienced buggy releases before though. Perhaps more to the point, the update is BIG, requiring around 6GB on the target system to run the install process - a bit tight on an 8GB iPhone or iPad, particularly if you actually already have anything on your device.