Joe Rogan is so popular because he let's other people be themselves. Live and let live. He doesn't try to enforce an opinion upon others. He sits back, enjoys himself and explores the depths of his guest by asking interesting and critical questions without giving much judgement.
People love watching the Joe Rogan Experience because they know that they will get to see the true self of the guest on the podcast.
In one moment Joe might say something which makes the audience think that he agrees with his guest's opinion, and in the very next sentence he might throw in something which might challenge the same thing they just said before. He's good at giving people a safe place to just talk freely and that is what people love.
He's done no harm to anyone, he's not using his platform to spread left/right wing views. He has guests from all corners of life and just let's them talk about their lives, experiences and views.
I love it.