To name a few examples why one would have that mentality:

  - Feature set changes (take telemetry for example with Windows 10, or changes in group policy options)

  - Unknown/untested compatibility with small non-corporate backed programs that a user needs to use

  - BIOS / Hardware issues (i.e. Power management changes and tweaks may push limits outside operating status)

Generally speaking while I agree with your vulnerability point, its not unreasonable to say new vulnerabilities can be introduced. Many people may value other things more which results in the mentality why one would ask or stay on a previous version of windows.