I agree with Bernie almost never (ironically, I still see more Bernie stickers that Hillary + Biden + Harris stickers) but he is correct. Harris was a horrible unlikeable candidate and ran a horrible campaign.
Zooming out the perspective, Dem leaders knew, with certainty, in summer 2023 (and probably earlier) that Biden was 4 or 6 eggs short of a dozen but feared Trump so much that they imagined they could drag him across the 2024 Election finish line (and maybe 25th-Amendment Kammie into the WH). So they didn't have a real Primary and were forced to do a bait-and-switch when Biden's debate debacle made publicly obvious that he was not capable of being POTUS for another 4 years (and probably should be 25th-Amendmented). It was July 2024, with the convention looming, so Kammie's was the one name with significant public recognition. Fear and loathing of Trump in 2022 and 2023 drove the Dems to this defeat.