They didn't fail

Nobody cared!

MS have done their utmost to push vista onto consumers, including blanket adverts on TV, Dell not allowed to ship XP (u-turned), and all the rest of the hype.

The reason its failing isn't because they didn't push it, its because people just don't want it. 90% of reviews go like this;

"The install took x long on my brand new top of the line y machine, so i had time to invade mongolia, and still had time to build a pyramid"

"Once I got into the desktop, it kept bothering me about security"

"It took me ages to copy a few of my audio tracks over the network, 3+ days"

etc. etc. etc.

People don't want vista, after 5yrs, everyone just wants a mac, at least they can deliver in the same year as they promise to.