Because the upgrades add technical debt without removing many of the problems of the previous systems.
My non-tech relatives all agree that windows 2000, vista, and 7 are all the same. Why make them move to something else the same? Security - what is that, when this was working fine before? ME was a disaster we would all like to forget - where do you expect their confidence to come from, in win10, when the past hasn't inspired any?
Why? Why would you NOT want to upgrade? I get that restarting in the middle of a skype call is shitty and ridiculous, but it has been a year now.