Windows XP was the longest used version of Windows ever to be released. Never before had nearly 6 years passed between two consecutive versions of a consumer release for a Windows OS update, so what happened is people got accustomed to the XP interface and the XP way of doing things and forgot what XP didn't do well.
Back in 2001 DOS games were still somewhat popular (although developers were finally switching totally to Windows totally at that point). But games that had only been released 2 years earlier would have massive problems running in the XP environment, and people really hated that incompatibility. There were some other problems even with Windows 98/95 based applications as well.
Talk about a lot of upset people, but people got over that. People are also getting over Vista's initial incompatibilities today.
Then came the Windows XP series of worm attacks that left virtually half the world's XP based PC's crawling to their knees in an automatic restart, many times too quick for the user to do a virus scan update, scan, and remove the worm.
I can't tell you how life was a living hell for such a long time because of this screen under Windows XP's original incarnation:
And that wasn't even a virus/worm you had to download anything to get it from, it kind of just popped up by browsing the wrong web site and your computer was infected.
XP had more technical problems than Vista does today, but people got used to it, and apparently people didn't get used to the new Vista interface as quickly.
Its also true that XP was released after an absolutely wretched operating system called Windows ME, so it was easier to love XP for its stability (even with major security problems like the automatic restart problem above, XP was more solid than ME).
But honestly, XP wasn't considered fully stable until AFTER the release of SP1. Only after XP had been released for a few years and they got the bugs worked out, and all patches applied did XP become the best version of Windows ever.
And I actually agree, after the initial bugs were worked out, XP became the most stable, most solid, most usable version of Microsoft Windows ever released. It was easy to fall in love with it after years of using outdated DOS based Windows operating systems.
Now Vista has went through somewhat the same thing, only Vista never had the restart screen of death, and its had far fewer technical glitches. More so just interface changes, incompatibilities with some older programs, and heavier system requirements.
So in all actuality, Windows XP had a harder start than Vista.
But 6 months is an eternity in the computer world.
People are realizing that newer OS's always take more system requirements and hardware, and people are also learning to appreciate Windows Vista's native 64 bit versions that are FULLY supported with a massive driver library. Windows XP 64 bit was always experimental and never "officially" released in many systems.
With the lack of proper driver support and the increased speed from a native 64 bit system, people are now coming around to Vista, especially when they install 4 gigs of RAM in their computers.
XP truly is looking outdated day by day, and its time people move on.