The problem for DNC was the absence of a clear frontrunner in a primary. The uncertainty may have been the deciding factor in peddling Kamala. Her losing to Trump may be viewed by them as a better outcome than the rise of a candidate like Bernie Sanders, who would have focused their platform more on progressive economic polices than social issues.
IMO, the Democrats’ most effective ticket would be a candidate who champions progressive economic policies with moderate social stances. Americans who are working, married, raising families, or planning to, don't give a shit about left wing social policies, and a large faction are absolutely disgusted by them, they just don't feel comfortable expressing it openly.
It seems that most people simply want the chance to earn a fair wage, access affordable housing, and raise a family in a safe community. Whether progressive economic policies can deliver on these goals is open for debate, but that’s not my point - my point is that this stuff matters more than any of the social nonsense they ram down our throats. Lately, it seems like Bernie is the only person on the left, at least at a national level, who seems to grasp this. Even though I don't agree with his policies, I do see it as an effective way to appeal to a lot of the voters the DNC has lost.