I think there are two different kinds of blackouts here. Most subreddits have opted to fully blackout by going private, but some have instead decided to not take the subreddit private but instead prevent new submissions and instead have a stickied submission explaining why no new comments/submissions can be made.

Larger subreddits might prefer the latter because it allows them to have one submission rise up through Reddit's /r/all or /r/popular, increasing visibility. Going fully dark by going private doesn't easily elucidate the reason as well as having a single submission doing so, and does a decent job when a number of the exact same submission flood the front page and nothing else from those communities.

This isn't to condone what a particular subreddit is doing or not doing but mostly opining about some of the ways different communities are participating