According to Cudmore on Twitter, he didn't come back because it wouldn't have been his voice and he would have been entirely CGI. He was basically offered to be a stuntman, or a stand-in.
Yeah I know but still. they would have still used his image. Doug Jones didnt turn down the Silver Surfer role even if they didnt use his voice.
His tweets saying that he doesnt know why his isnt in the film, if he really refused the role its a big mistake from him, I mean he owns the role.
Did Singer/Ratner want him to do an accent?