My pleasure! If I had to guess, I'd say he didn't have a good enough voice-- he was a specialist in Shakespeare and Shaw (yeah, tell it to Rex Harrison), and had few musical credits. He later did Guare's TWO GENTLEMEN OF VERONA, playing Antonio/Tavern Host--without much to sing.
I'll look at some of my stuff on Merrick tomorrow to see if the Abominable Showman had an explicit reason for the cut. But the spouse has already gone to bed--and I can't get at those books. :)