I don't think he said specifically why he did it that way, or I missed it if he did, but my theory, which is based on nothing, is that he is trying to preserve his voice by singing less. His voice sounded a bit hoarse last year at Morsefest at times, which I think he attributed at the time to doing too much in rehearsals in the days leading up to the shows, but I don't think he can go all-out every night, all night like he used to, and he is already singing less lead in his main band (Neal Morse Band). Neal is close to 60 now and is likely doing every little thing he can to save his voice for as long as he can. Granted, it's not like the Exorcist is an album for which he is going to tour, so he would have only had to sing it for the album and then at night 2 of Morsefest last year, but I like that he did it this way. Don't get me wrong, Neal is one of my favorite singers ever, but the fact that he takes a supporting role vocally on this album makes it feel a lot less samey than his work sometimes does, and when he does take a lead vocal, it feels pretty special.