You weren’t paying attention. Many states banned anyone else from the ballot, even though there wasn’t cause for that by the rules.
There were no debates (Something that would have given Democratic party members time to decide if they thought Biden was electable.)
Some states were told their delegates wouldn’t count.
There was no fair Democratic Party primary. If you think there was you were either not paying attention, or you didn’t want a fair primary in the first place.
This problem is a problem of the Democratic Part{EDIT}y’s own making.