The timeline forced the Democrats to go with Kamala. Since she was on the ticket with Biden she still got to access the campaign funds they'd raised prior to his withdrawal. That would not be the case for anyone else and those funds would have to be returned to the donors or given to the DNC if neither her nor Biden were the nominee. The DNC could spend that money, about $100 million, on ads but not on directly running the campaign as that would violate campaign finance laws. If the Democrats had speedrun a second primary or picked literally anyone else at an open convention, that candidate would have to build a campaign apparatus from scratch and start fundraising from a base of $0 all less than 4 months from the election. The Democrats really just did not have any other option than to go with Kamala since she was the only candidate with existing campaign funds and equally important an already existing campaign apparatus in place. You go to the election with the campaign you have if I can paraphrase an F-tier SECDEF.