Localytics poses this question. Why are Apple iPhone Users Slow to Upgrade?
I think this is the wrong question. It is way too generalized. All this study seems to show is a comparison between 3GS users going from iOS 3.x to 4.0 and Android users going from 2.1 to 2.2. When framed that way, there are several answers that seem obvious.
Unless you are synching your iPhone regularly, people may not realize there is an update. An OTA upgrade is more "in your face". The study actually does point this out and that was the point of this graph given the "Benefit of OTA upgrades" caption.
Next, one upgrade represents a move to a completely new version and the other is a point release. People seem more incline to jump right into incremental upgrades because the perception is that there is less chance that something major will break.
In a related thought, Apple's application ecosystem might have something to do with this. A full version upgrade is more likely to cause issues with applications already being used. A delay to allow developers the opportunity to get apps tested, updated and into the app store seems likely.
And finally, there is the matter of the return on investment. Is the time it takes to download and install the update and the risk that something will break worth the perceived benefits. There seemed to be, in general, a higher level of satisfaction with iOS 3.x than there was for Android 2.1. Add to this the amount of time between Android 2.2's release and when it was actually pushed out. It was clear that 2.2 offered huge improvements over 2.1 so that by the time the update trickled out, everyone was primed to grab it.
I love statistics (actually I love the graphical representation of data), but I'm not sure this is an apples to Apples comparison (pun intended).