Because he is not focusing on the content of his character, he is focusing on his accent and where he lines up on the color chart. It's racist, not to mention shallow.
It's not really an insult to Obama, but an insult to dark-skinned black men. Reid is saying Obama is one of their own.
He implies dark-skinned black men are not as good as their light-skinned counterpart.
I attended a small college. There were three girls graduating with a major in chemistry. One was a dark skinned black girl. She went home to meet her boyfriend's family. She heard later from her boyfriend that his mother and grandmother didn't approve because she was too dark for them and they wanted him to find a lighter skinned girlfriend. I've never forgotten that. His family would have been lucky to have her.