Joe beat Kamala in the Primary, he was on top and she was on the bottom (see what I did there?!). No idea how much shenanigans went on in the Dem primary, this may have been the plan way back then. Kamala is too radical left and has history, whereas Quid Pro Joe appealed more to the center (and his history, for the most part was under wraps). Biden was a better Dem nominee, had a far better chance in the national election. Dem Bait and switcheroo.