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? 馃檮) 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.