I think by posing the question you are overestimating the popularity of Obama within his own country.
Obama was a domestic failure; the deciding states were the ones that stagnated under his administration and only flipped red because Clinton was actively promising more of the same. 2008 Obama would have run rings around Trump, true enough, but with two terms of general dissapoinment behind him all he would have over trump was eloquence, and that didnt help clinton or the other republican hopefuls in the slightest.
Ultimately Sanders had a better chance of victory than Obama for the same reason as Trump's victory: he was a clear break from the previous status quo.
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