It's quite clear that WikiLeaks doesn't want to distribute via BitTorrent because they want control. Just look at how they handled the cables: they worked with media partners to analyze and release some of them. Later they totally screwed up and released everything unfiltered.
Since they want to maintain control they need people and bandwidth/hosting. Even if the the latter were free, people tend to cost money.
That doesn't solve control on its own... you still need (smart, trustworthy, courageous) people to go through documents, verify credibility, determine and redact dangerous info, deal with media/pr, not to mention legal issues. Tech people seem to consistently underestimate the non-tech needs even of organizations that are primarily software driven. I'd venture to guess that despite technology clearly being a core component of what WL does, it represents a relatively small percentage of their expenses.
You can distribute the encrypted version via BT and give the decryption key to the selected few to verify. This is no different from giving the unencrypted version to the selected few to verify.
Since they want to maintain control they need people and bandwidth/hosting. Even if the the latter were free, people tend to cost money.