Well Twitter lends itself to short sound bites, and has APIs for embedding tweets into a site that wishes to quote them. Makes it really easy for journos to report on what's said on it. Facebook has better support for longer posts, but still lends itself to sound bites a lot of the time. It's API isn't as good for embedding as Twitters, but it's useful enough, plus there's a lot of general integration for advertising purposes on Facebook. Reddit largely lacks in those departments. It's designed to link to places, not for specific posts to be linked to or embedded. It's designed around text, so lacks the same amount of in post image and video support of FB/Twitter and generally leans somewhat less towards sound bites. Also Reddit's division into subreddits makes it easier to see when someone's talking to a small audience, while FB and especially twitter you're left with the impression that it's being broadcast to the entire userbase.
So a journo looking to stir up outrage clicks can easily find a controversial tweet, embed it in their news article to be easily read, and call it a day. They can't really do the same for Reddit, so they just ignore it.