Why are there so many racist Chelsea fans?

I love football and I am a Barcelona fan. I also follow the Premier League very closely. I have heard of numerous accounts of large sections of Chelsea fans racially abusing black players in opposing teams. I know every football club in the world has a small racist and right wing followers who ruin the reputation of the majority of fans who are tolerant. But there seems to be something about Chelsea fans and racism, doesn't it? What really amazes me is that black players have been key to chelsea's success in recent years. Last year, Chelsea won the double for the first time in team history and the majority of the team's starting 11 were black players (Drogba, Anelka, Malouda, A. Cole, Essien, Kalou, Bosingwa, etc, etc). How can a team that has so many black players have so many racists? And What is the business of singing Anti-Semitic songs to Tottenham about? I know Tottenham have a large Jewish following but don't chelsea fans know that Anti-Semitism is a form of racism and bigotry? I am sure the majority of Chelsea fans are tolerant (I know a black chelsea fan) but there is really something about large sections of Chelsea fans that gives me the blues. I welcome a frank discussion and views.