Why? This makes no sense to me at all.
There are many reasons. Some don't like the intrusion and privacy issues that come with Windows 10. There are still a lot of unknowns as to what happens in July when the free upgrade is up, what will MS do to make money now? Others do not like the interface (although it is better than 8/8.1, and I use Classic Shell to give me the old interface). As for me, as I said above, I have 2 HTPC's that exclusively use Windows Media Center for TV, movies, music and all other media, and Windows 10 does not have it.
I have Windows 10 on my laptop and it is OK. I still like windows 7 best and is what is on my desktop in the family room. I have 8.1 on my main desktop, and am debating jumping in on the free 10 before it is up just to have the key if I want to keep it.