Porn = male gaze-y, intentionally and overtly about sexual gratification, less about exploration
A free-wheeling blog site = a place open to the exploration of a number of different aspects of identity, of which sexuality is one of them.
In short: If Pornhub were to create a tumblr clone, they'd inherently be enticing people to _produce porn_ rather than producing content which might happen to be somewhat pornographic. There's a huge mental barrier there for lots of people, even if they happen to enjoy viewing porn or producing sexually explicit imagery.
 Yes I know women watch porn, the default assumption of porn viewership is still a cis male audience