I’m beginning to think you might be trolling based on your responses. In case I’m wrong…
The simple answer to your question was that people who voted in the Democratic Party primary didn’t want him to be their nominee. Of course you’re asking why.
In 2020, Sanders had the lead and the party leaders decided “guys, we can’t run a Socialist Jew against Donald Trump, so we need to pick a candidate and go with him.” A ton of people vying for the nomination dropped out and endorsed Biden. Their supporters voted according to the endorsements and we ended up getting Joe Biden.
ETA: To be clear the Democratic Party is a private organization and they can do whatever they want. It’s completely within their rights to say “we need to stop Bernie Sanders” and put in action a plan to do just that.
Yes, ethically it’s a very bad look. But I’m not a registered Democrat (or anything else) so I don’t have a say in how they run their organization.