Definition of "porn" is too much different for Pornhub and Tumblr. For former, it's kafkaesque-lovecraftian movies with Hollywood-level effects depicting impossible forms of multi-person sexual intercourse, for latter it's usually just photos of women with uncovered breasts. I can't remember seeing "real porn" on Tumblr.
Pornhub is leveraged to overcome most of the difficulties in that market, which makes tumblr more valuable to them than it is to most companies.
Put another way, most buyers would proceed to (continue to) destroy a significant portion of tumblrs value in order to be willing to operate it. Pornhub would not.