1. Harris is a woman. In this country, that's a problem where leadership roles are concerned. When Hillary ran, women across the country responded that she wasn't fit for office because it was the office of a man.
2. Harris, as VP, has all but disappeared from the public consciousness (remember, conservatives, when she was the harbinger of doom? 🙄) She's been given no real responsibilities, as is tradition, so she hasn't been able to prove herself.
BUT, I personally believe if Harris can find her backbone (I'm not sure she's realized her full potential yet), she could prove a worthy and serious adversary. She cares much about public opinion from what I can tell (for what that's worth). Dems need someone unapologetic; someone who says, "Yeah, I get you don't see it yet, but we're going forward and in time you'll thank me." Only without the recent fascist bent.