Consumers don't feel many pain points that are harsh enough to change habits and loyalty from current market leaders.
Hard to monetize in scalable manner.
News (and even content) is percieved as small by investors.
Numerous - dozens - of failed startups, that have tried hundreds of propositions in the past decade - isn't clear what has changed to make those propositions work today.
Using someone elses content is of questionable legal grounds (EG: when you are winning, they'll knock)
Current social products like Facebook or Twitter or even Reddit do "good enough" job for vast majority.
