There is plenty of NSFW overall on Reddit, but AI-generated NSFW doesn't seem to be mentioned in any site-wide rules, in the Terms of Service and no reasoning is being provided by Reddit.
So why is this happening?
This is just plain not true. They very clearly ban AI-generated NSFW content, hence the removal of the subs:
>Additionally, images or video of another person posted for the specific purpose of faking explicit content or soliciting “lookalike” pornography (e.g. “deepfakes” or "bubble porn") is also against the Rule.
I Google'd "reddit rules", this[1] page was the top result, and under "Rule 3" the reader is linked to this[2] page, with the above quote.
[1]https://www.redditinc.com/policies/content-policy
[2]https://www.reddithelp.com/hc/en-us/articles/360043513411