Biden actively pilfered Bernie's platform after winning the primary, so he was a forward thinking progressive compared to Harris and Hillary. Harris in particular explicitly ran to the right of Biden; she wasn't exactly subtle about this. Perhaps more importantly, however, he had COVID on his side. Plenty of voters (33% of all voters and 57% of Biden voters) considered competent handling of COVID one of their two most important issues when voting; if COVID hadn't happened or had happened under Biden's watch Trump would've wiped the floor with him.