I can't talk about others, but for me, the number one reason is the UI. If I could be 100% sure that updating to a newer version would keep my UI just the way I know it, and that no workflow is going to be broken, I would probably update as soon as possible. I still do that with a lot of other software. On the contrary, Windows is very fond of changing everything whenever they can, and it becomes tiresome fast. For example, several versions ago (it started with Vista, I think), someone decided that updating the file association info and the commands associated with each file type could no longer be accessed from the file explorer. Instead, you need to access the registry. Cue the myriad of different small tools, each with a slightly different and possibly incomplete behaviour.

It's very telling that extensions like OpenShell and 7+ Taskbar Tweaker are so common. Also, I'm still salty at the removal of the Windows Classic theme and of the detailed. I use WindowBlinds as a patch of sorts, but there is still a lot of customisation missing. Customisation that used to be available in older Windows.