Turnout was lower because Kamala was not a popular choice with Democrats.
She didn't go thru the primary process.
She was a better choice than Biden but she was not the first choice for many moderate Dems like me.
Go back and look at my posts on her when she was announced as the nominee. I considered not voting at all. Many Dems didn't.
Biden was not a popular President, including with Dems.
She was seen as the right hand woman of an unpopular President.
The vast majority of Dems wanted him to drop out long before he did to make way for someone that could win.
She also lost the votes of Hispanic men, black men and white women who were not happy with inflation and the border.
Ask the majority of Americans how they feel about Harris or Trump. Most are not fans of either. That's why turnout was lower.