“Harris” is a more common name than “Kamala”, and “Donald” is a more common name than “Trump”. This is just my opinion, but I think Kamala is a more powerful sounding name than Harris, and that helps with her image as a stern prosecutor who wants to crush injustice towards women.

“Clinton” refers (in most people’s minds) to Hillary’s husband Bill moreso than Hillary herself. In her campaign, she leveraged her first initial for her slogan “I’m with Her” with the stylized right-pointing arrow in the H. For her, it seems to be her choice and more clear. For Harris, it just seems to be because “Kamala” sticks out in people’s minds more than her last name.