The optics of a potentially publicly traded company (what ever happened with the Reddit IPO thing?) openly having a "how to steal other companies copyrighted work" forum is more of a negative than IP farming with no method to actually do anything useful with the IP addresses.
Porn is less of a legal liability than copyright claims is my guess.
Also... like... if somebody is dumping money into Reddit as a user of Reddit, which is more likely to make them stop: killing the piracy forums or the porn forums?
Specifically, the executives and shareholders want their Publicly Traded Shares of company stock to be increasing in value. Advertisers are good for getting long term holders of the stock to keep holding and buying, but everybody else is looking to quietly pump up the numbers and sell what they can without upsetting the apple-cart.
I'd bet a shiny metal dollar that after the IPO starts, the porn subs will slowly start being winnowed down. Give it a few years and if any exist they will be private.