Internal politics is going to be responsible for some of it. This is an unexpected opportunity for individuals to advance their careers or agendas outside of the usual process, and some of them are going to take the opportunity. They might not even dislike the idea of Harris being the nominee, but they want to find a way to use their support to their advantage. The Democrats are hardly a monolith, they’re a broad coalition that barely holds together at the best of times, it’s not that weird that there would be conflict.
There’s also the issue that there hasn’t been any sort of democratic process to select a new nominee. Harris makes sense for a number of reasons, and the party does have the authority to nominate whomever they want, but they have to avoid making it look like the party insiders are just coronating a new nominee. It’s bad optics, if nothing else. This is also a pretty unprecedented situation, and it seems like no one knew it was going to happen for sure. It makes sense that there’s a conversation out in the open about who is going to be the nominee.
As a candidate, she’s not the best choice, but she’s an improvement over Biden. I doubt she would have won a genuinely competitive primary process. She’s probably in the best position to be the nominee at this moment, but there are no doubt plenty of people who feel that this could have been handled better and are going to make their opinions heard.