Why didn't Fredo become Don?

Hey Guys,

Did Michael become don because Fredo because Fredo was literally the weakest or because he took over the operations in Nevada?

Fredo was to busy banging the cocktail waitresses. LOL

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.