Perhaps because the level of respect that Windows has for its users has dropped with each successive version?

Not to mention bloat: I have a keyboard with a dedicated calculator button. On a machine with Core i5 something or other and SSD it takes about 2 seconds for the calculator to appear the first time I push that button. On the Core 2 Duo machine that preceded it, running XP from spinning rust, the calculator would appear instantly - certainly before I can release the button.

But also WinXP was the OS a lot of people used during their formative years - don't underestimate the power of nostalgia.

Also, for some people the very fact that Microsoft don't want you to would be reason enough!

Personally if I were into preserving old Windows versions I'd be putting my effort into Win2k SP4, since it's the last version that doesn't need activating. (I did have to activate a Vista install recently - just a VM used to keep alive some legacy software whose own activation servers are but a distant memory. It's still possible, but you can't do it over the phone any more, and I couldn't find any way to do it without registering a Microsoft account.)