For me it's because I just don't trust Microsoft anymore. Whats stopping them from using their broad powers under their EULA from collecting information about any device on my network that will respond to the Windows 10 machine's request? The amount of telemetry they force you to accept just to use Windows 10 is unacceptable. If I want a local user account without the OS phoning home for anything other than Windows Update, I should be able to do that. Instead, on a default install of Windows 10 Pro, which I bought a license for, I get tons of preinstalled crapware / links to install said crapware, a barrage of attempts to convert my local user account to a Microsoft account, an antivirus tool that's so aggressive that some of my software development tools have had to include directions for whitelisting their cache directories, and just this persistent worry that Microsoft will push out the next update and break my PC or start collecting new data, or re-enabling things I've already disabled.

My ideal Windows 10 setup would be basically Windows 7 + a wizard of 'Do you want this feature / telemetry' that actually respects my answers.