I think the disaster that was 8.0.1 prevented a lot of what would normally have been the second wave of upgrades. People heard "iOS update bricks your phone!" and didn't do what the little upgrade popup suggested, even after 8.0.2 resolved the issue.
There are also a lot of 4S phones out there, and quite a bit of press around the launch that advised people on 4S's not to upgrade.
I didn't upgrade my 4S, based solely upon Ars Technica's advice regarding the slowdown. Since then, I've weakened and bought an iPhone 6... so I guess I'll use 'Not upgraded to iOS 8' as a selling point when I flog the old phone on eBay :)
There are also a lot of 4S phones out there, and quite a bit of press around the launch that advised people on 4S's not to upgrade.