I've said this a number of times but at this point in the race the polls mainly judge name recognition. In May of 2015 the leading Republicans in the race were Jeb Bush and Scott Walker. Of course, Trump didn't enter the race until June or July, but Walker dropped out before any votes were even cast, showing that he didn't really have staying power. In May of 2007 the GOP front runner, by a large margin, was Rudy Giuliani. In the fall of 2007 Hillary Clinton had a lead in the Democratic primary which was described as "clear but not insurmountable". The primary polls this early out aren't very reflective of who ends up becoming the nominee.
The reason is that most primary voters aren't paying a lot of attention to the race yet. We're something like nine months before the first votes get cast in the Democratic primary. A lot of people just haven't given the race very much critical thought. They haven't really taken a look at the candidates, assessed their strengths and weaknesses, and made an informed decision on which ones they like. So when a pollster calls them up and asks "Who do you support for the Democratic nomination", they don't think "Well, Kamala Harris is bad for this reason and Corey Booker is bad for that reason, but Joe Biden is good for these reasons". Instead they basically just respond with the names of people they're familiar with. Since Biden spent eight years as a very well liked and successful vice president he's one of the best known Democrats in the race. The second place candidate, currently, is Bernie Sanders, and that's because he was the runner up last time and a media darling. Biden's lead on Bernie is likely reflective of many Democratic voters being unsure of Bernie.