Hype-cycles.
I remember some years ago being at a Gartner presentation where the guy said, in essence " any business that doesn't have a high-profile presence on Second Life is doomed to irrelevance in the next year "
I think we can all name a few businesses who took this advice to little benefit, and likewise plenty of businesses who didn't hear the message but for whom the Gartner-predicted doom is somehow taking longer to materialise than first predicted.