Not sure what Booker stood for or advocated.
His why-can't-we-all-get-along shtick and shying away from negativity seemed to read the national mood wrong. I think he was trying to mimic Obama in a post-Obama era after Obama's centrist, work-together message proved unrealistic. Obama got bamboozled over and over. The GOP was never going to work with Obama, but it took him 7 years to figure that out.
Otherwise, I think the US isn't likely to vote for another black president so soon, despite or maybe because of the current racial polarization. Realistically, black and female was two strikes on Kamala from the get-go.
After being mayor of Newark, rising star Cory Booker T had to persuade the wealthy donor class and NJ voters in general that he wasn't some urban radical, but a respectable politician. Newark is a complicated place in the Jersey psyche. It's the largest city in a suburban state. It's also been something of a war zone, ghetto, crime-pit, source of fear for a long time. There's been some success improving Newark with development, sport teams, a few big businesses (Prudential, major NJ law firms, etc).
Booker took some chances as mayor of Newark and was willing to try anything that might work. But he had to distance himself somewhat from the specter of Newark to become senator.
But cycling back, I have no idea why Booker ran/is running for president, except that it's the next rung on the ladder for an ambitious politician who has become a senator. And once you are senator from NJ, you're there forever, much like CA. In Senate terms, NJ is largely a one party state. And it's real tough to defeat a sitting senator in a primary. Even sleazy scandals haven't derailed creepy Menendez (D-NJ) who was re-elected last go while under indictment.
Recycling back, what I saw of Booker in the campaign, he seemed lightweight. I was disappointed he didn't seem to have much to offer or a strategy to get whatever message out.