What is the deal with Clinton delegates not falling in line?

I am seeking to understand why Hillary Clinton supporters seem to be hell bent on causing divisiveness within the Democratic party by not supporting Barack Obama's candidacy, especially after she herself has casted her support behind him? It seems obvious to me that the only way we put a Democrat into the oval office is by getting over being disappointed, and becoming united. Why would you want to divide the party and make it possible for the Republicans to win this November? I find it ridiculous that 27% of Clinton supporters will now vote for John McCain? Is it race? Gender? experience?