They don't want to deal with the legal implications of it. Spez has said ad nauseum that they don't want to risk 3PA providing NSFW content to users that Reddit is not allowed to serve because they don't want to be held responsible for that. Especially now that some US states are requiring actual ID verification for 18+ content.
While Spez is a lying weasel, I don't doubt that Reddit is worried about NSFW-related lawsuits, bad press, and ad revenue impact.
And, the next step after having control of the content is to further restrict it.
18+ /r/interestingasfuck forced open, lowers its standard of what qualifies as "interesting", gets flooded with adult content [NSFW within two clicks] (www.reddit.com)
I think the title speaks for itself.
Reddit: open /r/pics or else. Mods: OK but you didn't say how (old.reddit.com)
I honestly don't know if this is allowed here but I thought this is malicious compliance at its finest....