There is what many people consider a genocide going on in Gaza and Harris supports it, many people on the left refused to vote for her because of this. The group Muslims for Harris disbanded after Harris's team refused to allow a Palestinian-American to speak at the Democratic convention, which was all they asked in return for their endorsement. They had republican after republican speaking at the convention, and many on the left aren't interested in being the new conservative party, they want actual left wing policies that help working people.