They were not considered safe choices
The election of Donald Trump is viewed by many experts as a backlash against the cultural significance of Barack Obama’s presidency. The theory holds that the nativist rhetoric found among some in White America points to that very fact.
As such, Black candidates for the presidency would be riskier gambles. Their ideas would not matter, just the color of their skin.
More about this
This may explain why Booker and Harris were not popular among Black people. Bernie Sanders, a near-octogenarian Jewish man from Brooklyn, can count on more Black support.