Harris was the bottom polling democrat and was chosen as VP so they could manipulate the weak minded into voting for the dei candidate. 
Harris' polling numbers declined during her campaign for President in 2019. But she was not at the bottom of the heap. Michael Bennet, Cory Booker, Tulsi Gabbard, Andrew Yang and Amy Klobuchar all polled lower than Harris did. VP's candidates are chosen to compliment the Presidential candidate and to attract more voters. That she is Black/Asian and a youngish middle-aged woman undoubtably influenced her being chosen as Biden's running mate.