The only reason I can think of is some executive was so morally opposed to it that they value getting rid of the NSFW stuff on Tumblr over money. Twitter had no qualms taking in the manly Tumblr users who went over there instead.
From what I recall at the time it was because Yahoo wanted to go down the standard ad serving model to monetize tumblr and the presence of porn was getting in the way of that. They didn't realize or didn't believe that one of the big reasons a lot of people were on tumblr to begin with was for smut.
