Of course, you could argue that at least Obama has to be doing this, as he is by far the most extreme of those six major party candidates, and thus has the furthest to go. Add to this that the African-American portion of the Democratic Party is one of that party's more liberal factions.
Nevertheless, the distinction here is most likely that for those candidates moving outwards in order to attract their party's base, their chances are long shots, and thus it is far more important to them to get the nomination than to assume that, and move to the center. Hillary, of course, is still the overall front runner, and so it makes perfect sense for her to be starting to reposition herself to be more attractive to swing voters. And Guliani is popular enough to start doing this too. But why Obama?