Because it’s good for votes to show that a political party is willing to assign a position of high esteem like vice president to someone who is from a marginalized group, but not to a position of actual ultimate power, because that power should remain in the hands of the straight white men. Sure, if the straight white guy president drops dead, the minority female will gain that ultimate power, but we can deal with that situation if it happens. But we’re willing to risk that scenario for the minority and feminist votes for the straight white man we are running right now.
Yet Obama managed to be elected president under his own steam twice.
A real question is, (perhaps for another thread) is sexism stronger than racism in America?