A combination of the fact that most businesses won't upgrade for a long long time, and that most normal users only upgrade their OS along with their pc. As in, they only upgrade because they can only buy a pc/laptop with the new version of windows installed.
I can't comment fully on the business side of things as my knowledge of that is no where near qualified enough. But I feel I have a pretty good grasp of the normal consumer side.
I'm sure that given the choice, most normal non-pc savvy users would never upgrade their OS, and if offered, would always go for a pc with the OS they're familiar with installed on it. Even if it's XP.
The problem I've found, from working in large electrical retail stores and coming into contact with your average user on a day to day basis, is that new versions of windows never offer anything that is going to radically improve the day to day user experience from the point of view of someone that just wants word and an internet browser.
So why upgrade if there is no obvious advantage other than a fancy new start screen? It was the same story with xp to vista (most people saying "oh, it's a prettier version of xp with a thicker task bar and a gadget thing on the side") and with vista to 7 (although, more people were happier going to 7 as it was obvious that 7 fixed a lot of issues that people had with vista, but was nothing to do with new features).
Personally, I quite like windows 8, but only because I know all the tricks about getting the most out of it and where things have moved to. Every time I speak to a customer about 8, I find they have little to no knowledge about it, and are either upgrading an old pc or are buying new for a family member.
I rarely, if ever, speak to people specifically about upgrading their OS. In fact, in the entire 6 months I've been back in electrical retail, I think I've spoken to 3 people who specifically wanted to upgrade windows.
But as this is a small sector of the market compared to businesses I shall bow out for now. And yes, the company I work for still uses XP sp3 across the business