There's usually a certain percentage of those who are avid supporters of just one candidate in the primaries that don't show up to vote in the general. This is normal. They're the my candidate or none. Nominate my candidate or else I won't vote. They don't show up to vote for the other party's candidate, they stay home.

In the primaries so far this year and in 2016, most of Sanders supporters according to the polls are in the independent lean Democratic column, not those who affiliate or identify themselves as Democrats. Check out question 47.

https://docs.cdn.yougov.com/fcdckgt368/econTabReport.pdf

Sanders is the choice of Democratic leaning independents who vote in the primary 65-35. But Biden is the choice among those who identify or are part of the democratic base 65-29. Lot's of other good information there also.

Fact is those who identify with the two major parties are twice as likely to get out and vote then independents who lean toward one party or the other. Independents on November of 2016 made up 40% of the electorate, but only 30% of those who actually voted. Democrats made up 30% of the electorate, 36% of those who voted, Republicans 27%, 33% of those who actually voted.

The two parties base made up but 57% of the total electorate, but 69% of those who voted.

https://www.cnn.com/election/2016/results/exit-polls

So a bunch of Democratic leaning independent's who support Sanders or any other candidate to include those supporting Biden not showing up at the polls is normal in any election year.

It's high past time that we start electing Americans to congress and the presidency who put America first instead of their political party. For way too long we have been electing Republicans and Democrats who happen to be Americans instead of Americans who happen to be Republicans and Democrats.