Because race has been used to divide us since the beginning.
It has always been a factor in American politics...
And the reality is that it is the overriding factor in geo-politics also.
But in America, we really like to use it as a wedge issue...Obama is using it in a way too...So even if Obama is elected, the issue will still be there (unfortunatly).
I don't think all people are racist...But politics are all about factions...How do you divide, and at the same time bring together and manage factions to achieve political power...
Its making sausage.