The answer is in some ways much simpler: Biden isn’t doing well in presidential polls because almost no one of consequence in the Democratic Party, other than Biden, is talking seriously about his presidential prospects. Few Democratic elites have made public endorsements for 2016 yet, but the early signals are pretty clear. Priorities USA Action, the super PAC that threw so much money and effort behind Obama in 2012, has signaled its support for Hillary Clinton, and past Obama campaign leaders like Jim Messina and Jeremy Bird have fallen in for her. This is about as close as you can get at this point in the cycle to a full-throated endorsement from President Obama and the Democratic establishment. Potential also-ran Elizabeth Warren has the backing of some notable individuals, such as former congressman Barney Frank and TV financial guru Suze Orman. This isn’t much to build a presidential run on, but it does suggest at least some support for the Massachusetts senator, perhaps enough for a minor candidacy to advance some issues of importance to her. But who is backing Biden?
Suffice it to say that if a broad swathe of members of Congress and governors signaled their support for a Biden presidential run, his age, race, gender, and even the occasional gaffe would not be much of an issue. Older white men, even those who occasionally put their feet in their mouths, are still well represented within the Democratic Party. (Note the contested Senate races right now in Arkansas, Colorado, Iowa, and Michigan; all the Democratic nominees are white men over 50.) Obama pulled the occasional gaffe in 2008, and George W. Bush certainly had his share in 2000, but those didn’t really hurt their nomination prospects because so many party insiders were already behind them.