I don't think it's so much about race as it is about excitement. PBF has never been considered a terribly exciting fighter, and many of his fights didn't have much of a competitive element to them because he was far superior to his opposition. Pavlik is being pimped by HBO because he's a very exciting fighter who is willing to take on all comers, with a story-line behind him which many American's can relate to. No one can really relate to being raised in a boxing family and of being put in the gym from the age of 5, but many can identify with hardship and battling tooth and nail to overcome that hardship, whatever form it may have taken.
Exciting fighters bring more viewers and more cash to HBO, so they promote them (and whoever else is hot at the moment) in order to make as much money as they possibly can.