I hate to say this, because I'm sure the people involved were nice people, but asking why Pownce failed seems like the dumbest question I've ever heard. First because the company was so silly that even TechCrunch had trouble taking it seriously, and second because, as far as I can tell, Pownce didn't fail.
Pownce wasn't designed to capture any particular market or change the world in any particular way. Pownce was designed to be, and functioned perfectly as, a holding pattern for fortunate, privileged Silicon Valley/San Francisco celebrities who were, for a time, too well-connected to need real jobs. Northern California is full to bursting with similar companies. Ever so often times get tough, and even the well-connected need to produce something, so they fold up their inflatable castles and do some real work for a year or two, generating actual revenue, until the economy recovers.
It's sad, I guess, but it only ever happens in economic contexts where much sadder things are also happening.