I'm half joking, but I'm sure Biden was offered something in exchange for not running.
He was "offered" the opportunity to not end his political career with a humiliating 3rd place finish.
Come on, Biden would've done better than Sanders.
Yes. As much as I like Sanders, a lot of his power came from being pretty much the only opposition to Hillary. In Hillary v. sitting Vice President race the focus would be on them and Sanders wouldn't get so much exposure. Plus, Biden would be much more attractive for those reluctant to back Hillary who ended up supporting her due to having no one else.
I'm not saying he would've won the nomination (it would be probably a close run), but I definitively see him getting more popular vote support than Bernie did.