According to Apple’s own developer analytics, iOS 8 installs seem to have hit a hard wall, likely due to the free space necessary to extract the update. After three weeks on the market, iOS 8 is only on 47% of total devices, some 10% less than iOS 7 was at the same time last year.
The issue seems to be multi-pronged, but the primary roadblock is the up to 5GB of free storage needed to extract the near 2GB iOS 8 download. Users may not be willing to delete the photos and apps necessary to make the transition. (Of course, installing the update through iTunes requires only a comparatively minor 800MB-1GB of storage space, since the IPSW is extracted to the hard drive, not on to the device itself.)