Fredo didn't become the Don because he was too weak. We saw examples in the films when Fredo was drunk at Connie's wedding; was unable to protect Vito when he was attacked by Sollozzo's men; had a nervous breakdown after the shooting and was sent to Vegas to recover; was slapped around by Moe Green in Vegas, and took sides against him. And in II, Fredo can't even control the behavior of his drunken, slutty wife--and says so to Michael.

Very early in the novel, Puzo writes that Vito believed Fredo could never be the Don.