The only people complaining about there not having been a primary are Republicans.
The party is fine with her as the choice. They don't care and the vast majority of people online don't have a problem with her stepping in to take Joe's place.
They wanted the democrats to waste time and money fighting amongst themselves with 3 months until election day. The logistics would not have added up and there's no playbook for this sort of thing.
Anyone trying to ask this question is essentially asking "why didn't they fight amongst themselves? Why are they unified?" It isn't a bad thing that they're in agreement that this is their best move. It's obviously showing results as the poll numbers and amount of positive press and buzz online shows.
I can't wait to not have to think about this maga shyt again. I desperately want the republican party to purge itself of all these nazis and scammers and return to an actual party with policies that people other than bullies, religious extremists, child abusers, woman beaters, grifters and racists can get excited about.