Without looking through the responses (yet) I can guarantee the vast majority of any failed projects here can be contributed to a lack of focus on sales and marketing.
For us techies (and I include myself in this although I do sales and marketing for a living, so I've beat it out of me) it is far too easy and safe to return to what we know...AKA the technology.
I've looked through 3/4 of them so far, and most of the failed projects here are actually because there was no market for the product in the first place. Then there is designing the product wrong (so it doesn't do what the user wants, or doesn't clearly do so), not having enough resources, and not charging enough money. Lack of focus on sales and marketing is an issue in a few cases, and in those few cases it can certainly be a killer, but it's not the most likely reason for project failure.
For us techies (and I include myself in this although I do sales and marketing for a living, so I've beat it out of me) it is far too easy and safe to return to what we know...AKA the technology.