Patching together a stream when the people you follow are scattered across a hundred different servers painful. Account creation for average tech illiterate users painful.
This is a matter of execution. I think BrowserID is making this a lot easier. Also, we have an example of a decentralized communication system: email.
If they have to rely on some third party to provide a host, how is that any better than relying on Twitter or Facebook, as soon as the hoster decides it isn't worth the expense or to boot you for some random reason what happens?
If it's decentralized, I have more control. I use gmail, but I can keep a backup of my email and change my email provider whenever I want. Not so FB or Twitter.
I think Wikipedia's great, but I think its governance system is pretty whack, and I wouldn't want my communications to be subject to anything that capricious, undemocratic, or otherwise not in my own control.