Because the only 2 Demographics that matter; rural whites in the rust belt and moderate suburban Republicans are bound to dislike her.
Mmmk. If anyone actually wants an opinion from a clearly moderate voter who has given his vote to the Democratic ticket in one of the past two Presidential elections, I find her very arrogant. A lot of people said that about Hillary, but I felt Hillary's problem was more being "cold" and "aloof." Harris is neither of those things, and to me she comes across as spitefully sticking up for "Democrats" against the "rest of the world." If I were a Democratic partisan, I might really like her, but I'm not. And I don't particularly find her likable, certainly less so than both Joe Biden and Hillary Clinton.
Even Democratic partisans don’t seem to like her.