Because it definitely isn’t there yet, due to overrepresentation of white (cis, straight, Christian, abled) men.
So from a diversity perspective, she should have gone for anyone other than a white man, but instead she has to cater to white men who somehow still manage to feel like they’re lacking in representation, because picking a running mate that wasn’t a white man would have tanked her campaign. Everyone knows it, but few are willing to admit to themselves why - racism, sexism, and a bunch of others too.
Is he the best from the options she had? Looks like. But the options she had were limited.