Joshua has mentioned following reasons in his post.
1. Focus on social instead of personal.
2. They solve too many problems, or try to.
3. Theyâ€™re about making someone other than the user happy.
4. They sell it the wrong way
5. Not in it for the long haul.
6. They show too much of whatâ€™s going on, and get gamed.
7. They donâ€™t have an underlying business strategy of improving peopleâ€™s lives.
you can find detailed article at http://bokardo.com/archives/7-reasons-why-web-apps-fail/