One problem may be that Sanders has no heir apparent.

[MineralMan]

38. Sanders' Coalition of Supporters

is not uniform in any way. It includes malcontents from every corner of the political spectrum. From anarchists to extreme libertarians, his followers are united in only one way - they are discontented with the entire society that makes up the United States. So, it's no wonder than when Sanders fares poorly in elections, it is largely because his supporters simply do not participate in elections. They see elections a part of the broken system they want to disappear.

If they vote at all when Sanders is not on the ballot, they vote to disrupt the process. That is why Jill Stein and Gary Johnson got as many votes as they did in three hugely important states. Those votes came from the discontented.

Normally, they don't make up enough of the electorate to actually affect outcomes, but in 2016, they did affect the outcome, leading to the disaster that is the Trump administration.

Many of Sanders' supporters don't really support the Senator from Vermont. They support chaos. We see some such discontented people here at times, too.