Back before the 2016 Conventions Trump challenged Bernie Sanders to a debate. Bernie jumped at the chance and Cadet Bonespurs had to run away from his own challenge.
Bernie can control a room on power of pure charisma, force of personality and personal authority in a way few people are capable of. Just watch his Fox News town hall.
His policies are obviously great, we all know this. But you can't go into a hostile environment like a Fox News Town Hall and completely dominate proceedings on policy alone.