http://www.windowsvienna.com/ Has posted a reason for it:
Courtesy windowsvienna.comBruce Morgan from Microsoft, expresses his personal opinion:
"Blackcomb was the code name for some other project driven by other people, started many years ago. The people who chose that name and started that have moved around quite a bit since then, and different people have the leadership roles now. The vision for the product is quite a bit different, I'd imagine, than in the late 90s. So many things are different now that it's been many years since Blackcomb was the "next version of Windows after Whistler."