In 2015, in interviews, he made it clear the voters he wanted to attract were the white middle class.

This, in a nutshell, is why Bernie lost the primaries.

In an excellent article in Fusion, by Terrell Jermaine Starr, the reasons why Bernie lost the primaries are described:

How Bernie Sanders lost the black vote.

Based upon interviews with people that worked for the Sanders’ campaign it is shown that Bernie from the very start chose to ignore POC voters.

Despite desperate pleas from his black outreach team staffers, it was considered a waste to focus on POC voters by people high up in the campaign, because it was thought Bernie couldn’t win them anyway.