They sort of kinda did. You can post nudity now. Not porn, but nudity.
And the dirty little secret about Tumblr is, people were still posting porn, this whole time, they just weren't caught because Tumblr wasn't actually looking for it that hard.
Ad everything needs to die. It should straight up become illegal to advertise anything other than for people who opt-in. If your product, movie or service is good, it'll get popular through word of mouth.
Partially true: they want to bring it back but payment processors wouldn't like that (the post i am linking is from the CEO of Tumblr's parent company, who bought it from Verizon in 2019)
ostensibly it's because the porn industry is unstable financially, and connected to a lot of shady issues/illegal practices that could open them up to some kinds of liability. (I don't remember which sorts)