Because Bittorrent-powered P2P DNS is a weird buzzword, not yet really existing piece of technology. And, I'd guess, WikiLeaks probably want to publish (primarily) on the mass-accessible Internet, not at some obscure place where nobody except for crypto-geeks could access it.

They could try popularizing Freenet (which is already existing and is a fairly stable technology), but, again, I'd guess they probably have their hands full of other tasks already.

WikiLeaks hosts torrents. For example see the link "Click here to download full site in single archive" at the bottom of http://cablegate.wikileaks.org/. And, I believe, the content is already copied to (and being discussed at) Freenet and other similiar P2P networks.