Bernie Sanders hasn't wanted to join a political party and many of his supporters haven't either. Sanders, however, chose to run as a Democrat last year and the Democratic party is a group of individuals who've banded together to support other Democrats. Not independents. Since independents don't support all Democrats why should Democrats support independents? The Democratic party had rules that Sanders and his supporters chaffed at following. If he didn't want to follow those rules he shouldn't have become a Democrat. If he wanted to make up the rules himself he should've run as an independent or formed his own political party or like minded people. He chose to do neither.
The American political system has mostly been a two party system from the beginning. That means that one party often needs to get more than half the vote in each election and a party needs to get more than half in more than half the elections. The parties have always had firm, usually opposing, positions on issues. And they've operated on the idea that no one needs to adhere to all those beliefs, that people in their party need only support something in the half to two thirds of those positions. There's never been any strict requirement that to support a party you had to have a particular stance on an issue.
And there's good reason for that. Take abortion. While the numbers have varied a bit, about half of America is pro-choice and the other half is pro-life. If everyone who is pro-choice voted Democratic and everyone who is pro-life voted Republican each party would win roughly half the elections. That'd work fine. But pro-choice voters don't all support the Democratic party.
As you can see in this poll 36% of Republicans support abortion being legal in almost all cases. So Democrats aren't getting their votes. But that balances out, because 24% of Democrats think abortion should be illegal in almost all cases. That's because these people agree with their political party on most other issues. Democrats wouldn't win that many elections without their pro-life voters.
That's why it's perplexing that Democratic National Committee chairman Tom Perez declared, “every candidate who runs as a Democrat should do the same, because every woman should be able to make her own health choices. Period.” The logical conclusion from what Perez is saying that every Democratic candidate has to be pro-choice and, by logical extension, every Democratic voter does too.
This is a suicidal party position. If voters who are pro-life even vote for independents, and not Republicans, Democrats are going to lose most elections. They'll be a small minority in the House, Senate, and won't win the Presidency. Because there are only a few places where people who are pro-choice and support Democrats are the majority of voters. I can't imagine this is a path Democrats will go down.