Why? I really enjoyed the book, over-colorized, probably written to be made into a movie. He never portrayed SBF in a good or bad light, but he showed the mess SBF and his companions made.
Lewis wrote about and gave plenty of interviews outside of the book about how incredibly inciteful SBF was... basically up until the moment of his arrest.
I got the impression of an "Elizabeth Holmes is wonderful" vibe from those interviews, showing how people perceived SBF. The overall image of SBF from the book is pretty bad, but it's never stated directly.