Ambition and rumours.

There was so much hype for how amazing "Longhorn" would be, so many overstatements (many of which never originated from Microsoft, though some did) and so many stunning concept videos that simply weren't ever going to be reality. Then features started getting dropped or scaled back (which was really only an issue because people had heard of them) and then there were long delays. The interwebblogs crowd starting to declare Vista a failure, the parroting tech journalists followed suit and declared Vista a disaster and then people who'd never even thought about it decided they'd all heard so much about how it was a disaster that they didn't want anything to do with it.

None of it necessarily has anything to do with the actual product called Vista at all, but that's kind of beside the point.