I dunno don't get me wrong Ethel Merman is one of the most talented performers to come from the Broadway stage but she's was so hard to take in when it came to movies. I can't even think of the proper word for her. She is one of those performers that is larger than life, and while that might work the stage that doesn't necessarily mean it will work in a movie. You gotta remember she didn't even do the movie version of Anne Get Your Gun which was done about 10 years earlier, and that was another mega hit on Broadway. I thought Rosalind Russell did alright even if she was dubbed for a majority of the movie. I do think Judy Garland could have done wonders with the role.
Merman was not a movie box-office draw, and it was not felt she would draw ticket-buyers in small-town America. Warner Bros. used the same judgment for casting Audrey Hepburn in MY FAIR LADY and Lucille Ball in MAME - all three are casting decisions over which fans of the shows are still shaking their heads in wonder. But these were business decisions made by businessmen.
Funny you mention that Harold Robbins, I was thinking about this other day. Warner Brothers knew how to pick great musical stage properties for movie versions but when it came to casting that was a different story. The only time they probably got it right was with The Music Man with Robert Preston during that era.
They got it right with Robert Preston because they retained him from Broadway.