Be fair to both candidates: ignore everything they've said and done prior to this campaign and really take a fresh, clean look at these two men based primarily on what they've said and done (position papers, debate performance, the whole lot) in this campaign. I think they sum up the values of the two groups of voters really very well.
So why didn't they do better?
In the case of Edwards, first off it's pretty amazing that the good-looking white guy in the field -- now with added populism, no less -- couldn't find traction. It says a lot about how eager Democratic America is on changing the image the US projects to the world. I was ready to vote for Edwards in the Florida primary, now I'll probably vote for Obama. (please note that this is not any indication of who I'll vote for in the general election.)
I think Edwards' problem -- in the main -- was that he's too slick. Romney runs into this as well, but Romney comes off as saying anything to get elected, whereas Edwards does appear genuine in his new embrace of populism. His message, now fine-tuned, is the one I most wanted to hear -- but he couldn't overcome the excitement that the Democrats might make history (in a number of ways) with either of the other two candidates. It's what's turning people out at the polls (in all contests so far, Democrats have way outnumbered Republicans voting, even in strong Republican states).
As for Rudy -- his problem is simple: he represents the ugliest side of the Republican party. He is fear and paranoia and authoritarianism -- and nothing beyond that. The more he campaigned, the further down the polls he went. The idea of Rudy was appealing to Republicans (a lot like the idea of Fred Thompson), but the reality was
More than that, though, I think a significant number of Americans from all walks of life are pretty tired of anybody they've seen before. This would certainly explain why Hillary is having to actually earn her nomination instead of cakewalking to it. It would also explain why it took McCain so long to get momentum going (and the early success of mentally-ill Mike Huckabee).
McCain is certainly the hardest of the Republicans to beat, though he will ultimately lose if he's the nominee (and so would Romney if he's the nominee -- the early numbers tell the tale, Democrats are chomping at the bit to vote and Republicans aren't really very excited about either leading Republican by comparison). If it's Clinton v. McCain, it will be close but people will hold their nose and vote Clinton (this is no slight on Hillary's ability, just we're freakin' tired of dynasties!). If it's Clinton v. Romney, it's a cakewalk. If it's Obama v. McCain, it's Young Hopeful v. Old Tired, and Obama wins. If it's Romney v. Obama, it's Creepy Phoney v. Slick Sincere and Obama wins. He generates optimism and excitement, and the very beat-up, bloodied and broke America George *spit* Bush has left us with is desperate for some optimism and excitement. Think of Obama as the Democrat's Reagan (in terms of being able to generate hope despite lack of substance) and you start to "get" his appeal.