That's because it's offered voters an insight into his character that isn't flattering. For most of his grown up life, the chips have always fallen his way -- whether it's been his state senate campaign, his US Senate campaign, or his presidential primary campaign. It's easy to seem like a nice guy when people are acting like you'll be elected president by acclamation.
The test of character, however, is how people respond when the chips are down. We know how McCain responds -- he proved it first at the Hanoi Hilton, refusing early release. On a lesser scale, he proved it again in his determined, persistent and ultimately victorious primary campaign.
Now we're learning how Barack responds when, for the first time, his campaign isn't clearly ascendant. It's revealing, but it isn't pretty (wasn't the press telling us that McCain was the one with the temperament problem?!). Presidential campaigns are tough and sometimes ugly. Maybe that's a good thing, because serving as President isn't a tea party. If a guy can't handle the pressure of being down a few points in a poll, how, exactly, does he propose to lead America (and the rest of the free world) for the next four years?