Biden was a lead weight that she could not overcome.
He pooped the bed on immigration, he severely lacked any ability to sell any of his successes (let the opposition control the narrative when he indeed had significant successes), and he went back on his pledge to be a “caretaker” POTUS resulting in severely handicapping her ability to run a campaign with barely 100 days.
And, when given a chance to differentiate herself from him, she failed.
She had her own issues, as all candidates do, but the largest fault of her campaign was her boss.