I think it has to do with racism. Williams sisters have and are still dominating a game that was predominatly white for many years now. Remember, when the sisters came on the scene nobody saw it coming so they thought this was just a one game win when it started but it has gone on for years and the hating gets worst because the best white female players that they have promoted so hard and pushed so hard keep fading with the sisters eg. sharapova. Since they can't talk about how weak and terrible their games are, since this is a sports you have to win, their looks, behavior and strength becomes the only tool.
These are people, who if you notice stays to themselves because they have each other and many people hate that. They are not doing drugs, prostituting themselves like many others eg Jennifer Capriati, Martina Hingis got so mess up during their time) and this is unbearable for them they must be animals becuase this is inhumane then.
Bob Marley said in one of his songs"You are going to be tired of seeing my face, but you can't get me out of the race" and that is it with the Williams sisters. People can't take them anymore because they win when they want to and doesn't care who they play and are open about it. Serena is right when she said"no one is going home happy when they see their name next to mine" who would be, knowing they are many times going to be humiliated.
Williams the father is a different story. He was born in a different time and have experienced what Americans are at their worst and have openly spoken about it and that hurts because many Americans have never met anyone who is of another race who move on the scene to make changes and who doesn't care a damn about their rules etc they are here to change the game in every aspect and they have.