Why is Vista so buggy?

This question keeps appearing in this and other ng's. I found the following
to be a very apt description of why bugswatting in Vista is so tough:

"Compared to the initial release of Windows XP, the sheer amount of
different hardware to be supported, as well as the almost infinite
combinations of hardware is proving to be quite a challenge for Microsoft as
well as the hardware vendors. One of the biggest hurdles to be faced lies in
the fact that it is impossible to find bugs, when certain hardware does not
work...."

I took it from PROneTworks Vista Newsletter Vol. 1, Issue 55, just released.