The US President Joe Biden's decision to continue his election campaign until July made it practically impossible to hold a primary election with multiple candidates.
This is stated by Nancy Pelosi, former Speaker of the House of Representatives for the Democrats, to New York Times :
If the President had dropped out earlier, it's possible that we would have had other candidates in the election campaign, says Pelosi.
She adds that she believes Kamala Harris would have been strengthened by such a process.
But we don't know that, it didn't happen. We have to live with what happened. And since the President immediately backed Kamala Harris, it became impossible to have a primary election at that time. If it had happened much earlier, it would have been different.