Partly because of discrimination, partly because she lacks charisma. There are a substantial number of people that dislike her because she's a black woman and they have biases against both, sometimes without even knowing it. There are also some people on the left that dislike her because she's a moderate liberal that used to be a prosecutor. Honestly she's about standard as vice presidents come, so though I'm farther to the left than her I don't have any strong feelings on her.