Biden gained this big lead in a part of the campaign that didn't have a bunch of debates. He ran on electability, and a lot of people in the party endorsed and embraced him on that basis. Now that Sanders is on stage with Biden, Sanders is trying to create a debate around policy more than electability and argue that he got the policy decisions right in the past and therefore will in the future. That seems perfectly logical for Sanders -- it's a way for him to illustrate why he has running for president but not really hurt Biden. Biden is not afraid of the idea that he is to the right of Sanders.