The US is deeply divided and the Democrats are deeply divided. Harris doesn't have anywhere near the charisma of Obama or Bill Clinton. She will be elected on not being Trump. Take for example how she handled protests against the continued support for Israel at the DNC. She outright told protestors that they shouldn't talk about it, because that helps Trump and she denied any Palestinian-American to speak, because the party is dependent on AIPAC.

She simply cannot be the president of the "entire country" at the moment. Addressing the rifts created by Trump, Hillary Clinton and the like takes time. Also she will need to purge a lot of people from the Biden establishment before being able to run on her own platform.