Because when idiots hear Socialism they see Communism and it's frighteningly easy to start a red scare propaganda campaign to keep him off the big ticket.
The guy who fixes my car basically runs a French-style salon out of his shop, and he told me, "The South Will never vote Socialist." Seems he's right so far. That and he goes against the powers that be... Bernie would have been a nightmare for the rich and powerful.
He had 4 years to shore up his weakness with black southern voters and never did. He could've offered Bootyjudge VP in exchange for staying in the race and keeping the center vote split but didn't.