Windows XP was a downer for many when it came out because it wasn't Windows 98SE++ (and Windows ME was a flop) - it was Windows 2000 with a "fisher price" skin.
For those in the know, that was great. Windows XP + classic theme + drivers (even if you had to find the W2k drivers) was incredibly stable and performant.
But it took a few years for games to catch up and get the updates, and so for the "tech oriented" crowd (which were often gamers), it took a bit to catch on.
But its longevity is a tribute to how "good it was" and how hard it was to improve from it. Vista introduced more stability at the expense of requiring new drivers, so they re-released Vista a few years later as 7 (there's really no major differences between Vista and 7 except a few years of upgrades to drivers and hardware).
The five years between XP and Vista certainly didn't help matters, either. If you wait that long, you better be willing to support it for decades. Windows XP's last update was 18 years after it was shipped.