8.1 is not an automatic update. It requires launching the store, accepting the update and waiting for the lengthy download and install process to finish. I have seen plenty of Windows 8 PC's that nobody bothered to upgrade. Not a single person I have talked to still running 8.0 was even aware of the upgrade. It's not like they made a conscious choice to stick with 8.0, they simply didn't bother to even find out. Microsoft would have to make a greater effort to force them to upgrade through automatic update and continuous prompts that keep requesting permission to download and upgrade when they boot up to get this to change.
That's pretty much the one and only reason why most of these users have not upgraded on their own. 95% of those windows 8.0 users are simply not clued in to the fact an upgrade should be done. 4% likely had problems getting the upgrade to install or download so just stick with 8.0 rather than troubleshoot the issue. Lets peg 1% or less are those choosing to stick with 8.0 (good enough for them, corporate standard, too much trouble, not enough bandwidth to download, etc etc)