Booker have plenty of personality when you see them on interviews you just don't like them because they aren't outspoken progressives
No one cares if he can give a good interview. There is nothing extraordinary about the way Castro speaks. Why do you like him specifically besides the fact that he's young, Latino, and charismatic? He has a "progressive" agenda? Hillary had a "progressive" agenda and it didn't help her much. I would even go as far as to disagree with the notion that he has "plenty of personality." I'd argue that he has little to no personality. He's a perfectly groomed and polished politician. It's almost creepy.
Any politician that was strongly associated with Hillary Clinton is going to have a tough time garnering votes from the Sanders wing of this party. Castro is one of those politicians.
He violated the freaking Hatch Act in order to tout Hillary Clinton for Christ's sake!