Stats from netmarketshare.com and other web statistics sites are showing very low upgrade rates for Windows 8.1 with roughly two out of three users staying on OS 8.0 instead.
With only 37.3% of users upgrading to the newest version of Windows 8, the question is why? Version 8.1 is a free upgrade, and is not subject to the carrier approvals that typically cause issues with upgrade rates. There is an understanding on why users may want to stick with Windows 7, but for those already on 8 there is no reason to not want to upgrade to 8.1 instead.
The culprit may be the manner in which Microsoft is pushing the update. Instead of making the upgrade available through Windows update (as they have traditionally done with service packs), the upgrade to 8.1 must be downloaded from the Windows app store. This process is actually very smooth, but the suspicion is that users simply are not checking the store often enough to notice the upgrade.