Who are the "Democrats" that you are talking about? No one held on to Joe Biden, he was president and decided he was going to run for re-election. No one seriously ran against him because people generally don't try to run against a sitting president in a primary and if they had they would have almost certainly lost in a landslide. Biden didn't need anyone's permission to run, and no one could have stopped him from running. Then he dropped out and endorsed Harris no credible person attempted to run against her. There is no secret party leadership who decides on the candidate that we can now blame, and besides some of the efforts to convince Biden to drop out everything was done very publicly.