Late to this party, hope OP gets to read it.

I think there are a confluence of factors with Bernie and his lack of appeal with minority voters, so I'll list a few out:

Working for vs. Working with Minorities

Many Bernie supporters state that his policies would help minorities, if they were aware of it, so they're not sure why minorities don't like it. Aside from that attitude being somewhat paternalistic, and off putting to many groups ("My plans work, so if you knew better, you'd be supporting me" isn't winning hearts and minds with them), it's basically suggesting that Bernie is working for them.

That, however, isn't the same as working with them, which is what minorities want. They don't want someone doing their job for them - they want people that work their communities and community leaders to solve problems together.

Lack of Ties

Referencing Bernie having been in the Civil Rights Movement is great, but the question is, what have you done for me lately? Bernie has represented Vermont - one of the most white states in the US - for decades now.

Where are his ties with black and Hispanic community leaders? Has he supported young Hispanic and black Democrat politicians elsewhere over the years?

We laugh at Jesse Jackson and Al Sharpton, but they're important people in the black community for a reason, and that's because their leadership matters to many, and their endorsements matter to politicians. He doesn't have the history of close relations with them that the Clintons do.

Conflating Democrat Voters with Liberals

Here's another big issue - not every minority group is liberal, even though most vote Democrat. Bernie's base supporters identify as 'very liberal' according to the Iowa caucus. He, however, lost the 'somewhat liberal' and 'moderate' categories to Hillary - which is where most minorities identify.

Take the black community, for example - church matters a lot to them. On social issues, they are often conservative - in 2008, it was the large black voter turnout in California that saw gay marriage banned and legalized marijuana fail the ballot initiatives there.

Similarly, not every minority group benefits from Bernie. Asian Americans, for instance, are overrepresented in financial jobs and many of them are known for saving money to send their kids to college. Paying for others to have free college, after working hard for their own kids and expecting taxes to be raised on them and their jobs, isn't particularly appealing as you might imagine.

Just some food for thought