I think a big part of this is a combination of two factors: First, most non-tech people never ran Win2k. They went straight from 98/ME to XP. Second, no one remembers WinXP before SP2. (You know, when it was a complete trainwreck.)
FWIW, when I was doing software development in the latter days of XP, I found that I actually liked the style of the UI widgets from the default theme. Its just the titlebar and taskbar styles that I couldn't stand. However, some size tweaks made them more tolerable.