It happens with every version of windows, some people are just happy with how things are and refuse to change because it works perfectly fine now. If that wasn't the case, we wouldn't still get questions about XP 19 years after it was released.
Another reason is age of PC, there are plenty out there that came with vista that would still run win 7 fine but start to struggle running win 10 mainly as hardware makers haven't supplied win 10 drivers. There aren't many of those but there are enough. XP pc are even more in that realm. I wouldn't have updated my last PC to Win 10, I didn't even look at WIn 8 with it, but i had other reasons to ignore WIn 8.