Biden was chosen because he was Obama's VP and, as such, they could attribute the overwhelming voter turnout required for his victory to Obama. Any of the others would have made the cheating more difficult to dismiss and had it been Sanders, they couldn't have dealt with record numbers of Americans turning out to vote for socialism.

There wasn't anything more to it at the time. After that fact there's his grotesque family and poor health that makes him easy to control, but first and foremost they needed his history to occlude Trump's "defeat".