Because they know that Obama is just a stalking horse for the real candidate: Hillary.
Would the RATS really nominate Hillary instead of Obama in Denver? Wow, that could be a big mess.
I'm not sure they'd do it. According to CBS,
Obama Hailed By House Dems. Every one of them is a superdelegate.
And if they did do it, nominate Hillary in Denver that is, she'd be broke (in debt), black DemocRAT voters would be demoralized, and that speech at Mile High stadium would look pretty silly. And then there's Bill.