It appeared to me that at least 99% of the porn on tumblr was not original content, it was commercial porn upload by random people in violation of copyright law.
They must have had to deal with a very large number of DMCA takedown requests.
Even with automation that takes down every blog entry that is reported through a web form, it seems very likely to me that just the staffing cost of dealing with emails and paper letters would outweigh the meagre advertising revenue from porn blogs.