For this discussion's sake, there are two forms of communication: (1) The public forum you can meet up with new people and talk. (2) And the direct message sending kind like phones and emails. Though the NSA is actively monitoring both, here we're discussing the latter.

With that out of the way, they're targeting the former in-order to make it impossible for people to come together and organise privately and anonymously around subversive ideas.

The specifics of targeting IRC are probably tied to the efficiency of the protocol which allows very cheap hardware and minimal bandwidth to the extent that non-complying private foreigners may provide a free forum the government can't control.