Bernie polled better during the primaries than both Clinton AND Trump. In fact, there were polls showing that he polled better than Trump among Republicans (so long as you only talked about his policies without mentioning his name. As soon as you said his name they'd call him a dirty communist and 180 their opinion - quite literally going from saying they'd vote for somebody with those positions to vowing to never vote for him). Clinton polled worse than Trump, and Bernie had a decent lead over Trump - enough that he was considered the better candidate to run against Trump right up until he dropped out of the race.

So, what happened? Well, major news networks airing 30 minutes of Trump's empty podium instead of Bernie's speech happened. He was the target of a major campaign by the leaders of the party who poured tons of money into making sure Hillary's face was everywhere and his voice was snuffed out. They quite literally said that they were under no obligation to run a fair primary.