Why did populism work (of course we use 'work' in a relative sense since Obama did win the nomination) for Edwards and then Clinton? Because, for all of Obama's great gifts as a politician -- he may well be the best political orator in the past 50 years -- his "common touch" still needs polishing.
(Obama's) natural approach to any issue is professorial; talk to smart people who are familiar with it, develop solutions and then use his political skill to sell voters on it.
The Fix still remembers well a moment at the MTV/MySpace forum last year in Iowa when a young woman, fighting off tears, told of her father being deported. Obama responded with a clinical breakdown of the problems with the immigration system in America but offered little empathy for the woman.
In that same situation, it's easy to imagine former President Bill Clinton wading into the crowd to hug the woman, his eyes misting up as he railed against the injustices of immigration laws in America.
In some ways, Obama's resistance to that sort of empathy politics may be directly related to his resistance to follow in the footsteps of the last Democrat to be elected president. Obama seems preternaturally aware of exploiting emotion for political gain -- an admirable trait in a person but not necessarily the savviest of political strategies.
McCain's decision to embrace populism on the stump is not only a nod to the successes (limited, of course) of Edwards and Clinton during the primary season but also a recognition of the current political environment.