I think over the next decade or two this traumatic brain injury issue is going to become a huge deal for MMA, boxing/kickboxing, American football, and some other sports.
I would imagine Rogan (being a compassionate guy) has been struggling for a while with ethical concerns over his role in MMA in light of his knowledge of what it will do in the long run to at least some % of the fighters.
He did the right thing by talking about this publicly and in a context that adds a human face to the issue (Schaub).