First off, I think the media played up the racial issue around the SC primary, and Obama did disappoint me a little when he said that Clinton's MLK/LBJ comments were "unfortunate" and "offended some folks". I think Obama should have said that the comments were not racial in any way; however, he did not say that, he wants to win--whatever. I also think Bill Clinton should not have made the Jesse Jackson comments, but the comments had no impact on that election, he made the comments on election day and Hillary had given up on SC days before, so the polls were already showing African Americans were going to vote for Obama in big numbers. Also, Jackson is really the only person that won multiple primaries and SC and did not win the nomination over the last several decades, so Bill Clinton was just trying to minimize his victory.
Now for why Obama is winning, he basically has Howard Dean's base plus African Americans. Also, he has been able to get the Deaniacs to the polls in greater numbers and I think this has to do in part with Facebook and other social networking sites. Howard Dean is generally compared to Bill Bradley, Paul Tsongas, etc. these people are seen as progressive/reform candidates but generally their campaigns go nowhere. So why is Obama's campaign going somewhere? A huge difference between Obama and the other progressive/reform candidates is that he is a great candidate, Dean, Bradley, and Tsongas were pretty bad candidates and Bradley and Tsongas are very difficult to listen to. So generally the progressive/reform candidate generates enthusiasm but fails, so the progressive base is not enough to win.
Obama also staked out the middle ground in 2007 to better position himself for the general, he did this while maintaining his appeal to progressives, this is a pretty good feat, see John Edwards for what happens when you feel the middle ground is taken and you have to run to the left.
Now another big difference between Dean and Obama is that Obama appeals to African Americans--without the large Southern states Obama's campaign would have been over on Super Tuesday. Generally, the establishment candidate, not the progressive/reform candidate, gets the African American support. Now in the past African American candidates ran but did not get African American support, so it is not just that he is African America, see Al Sharpton and Carol Mosley-Braun. Obama won Iowa and came close to winning New Hampshire, that made him a viable candidate. Also, the war is extremely unpopular and the overwhelming majority of the members of the Congressional Black Caucus voted against the war (Obama represented a district with a majority of African Americans and so opposing the war in 2002 was politically expedient for him).
So other factors have to be present for Obama to get African American support, so just being African American would clearly not get a candidate 85% of the African American vote. Obama is African American and he is getting 85% of the African American vote, but clearly not every African American candidate will get 85% of the vote.
Also, this is why demographics and momentum are so important to this race, because other than Wisconsin, demographics have controlled how a state votes. Also, it appears both candidates' bases win them substantial amounts of votes.