Well I guess its just because when he was elected he didn't have much in the way of experience or credentials, leaving his major campaign point being that he was black, and a Democrat like most other blacks, whereas blacks who ran before, and after, him were Republican.
And I'm fairly sure Republicans hate him for all the reasons they list.
Although I won't deny that there are quite a lot of idiots who just hate him because he's a nigger, but those guys aren't in Washington.
Well, maybe a few are.
But there are racist Democrats too.