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 t

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.