The most obvious reason for this poll is that Harris' post-nomination honeymoon phase is over and the public is beginning to see her incompetence. As she braces for her first debate, she won't only deal with the fact that Trump is better than her on immigration and the economy, but that Trump is statistically likely to beat her should he remain within a point or two among voters.
Harris, as VP-turned-presidents like LBJ have done in the past, has been forced to take heat for her boss's failures but receive no credit for his successes. Also like LBJ, she has an opportunity to show that she is her own candidate with new and popular ideas. The American people no longer care about VP Harris—they're eager to hear How Pres. Harris would uplift the country after defeating the bitterly partisan Trump campaign.
There is a 52% chance Vice President Kamala Harris will be elected US President in 2024, according to the Metaculus prediction community.