He may not have been 'fit' to be Don, I agree.
But after Vito was shot everyone seemed to be following Sonny's orders pretty well. He was the Don and to be treated so until his father was recovered.

Of course, Sonny's death (due in part to his being 'unfit') forced Vito to take back the reigns a little earlier than anticipated but then the return of Michael took care of that.

It's possible that Vito knew full well of Sonny's flaws and was grooming him to keep a more level head in anticipation of running the Family years in the future. He may have been successful but for the shooting...and Sonny was forced to take over long before he was really ready.

Apple