Why people are still using windows XP even after launch of windows 7 and 8 ?

Why does even after windows vista, windows 7 & windows 8, still windows xp is still in more use.
This is what my observation..

Many are using because they are happy with it, few others are using because they don't want to spend money to upgrade. May some are just using it because they don't have any problem with it.

I think also some are using it because their computer are not advanced enough to run latest windows.

Are you seeing people using windows XP , is it office or school ?

I mean to say office.
Even if someone prefer me to get an desktop or laptop with latest OS. I will say XP is better for me.
I think main reason is understandable.

Personally, I think XP was as you said, understandable and more on terms, user friendly. I preferred it's soft design more than that of WinV/7's sleek, smooth and Aero designs. And besides out of all of Windows' OSes XP stood out most easier, friendly and compatible. But right now, several programs demand the use of a newer OS, so maybe in your office, they are using minimum funds or maybe the software and information they want, can be accessible through WinXP. Alongside, I think WinXP is as fast as the newer ones. So to explain something.

XP was good but even windows 7 is also good, I think its just that you were so much used to it. Now I find Windows 7 more easier to use because certain new features. Earlier like you I was also in favour of XP but now I find 7 good.