The play only has the characters of Frankie and Johnny, it takes place in Frankie's apartment in one night. So obviously as movies often do, they had to open the story up to more characters and more locations.
The controversy at the time wasn't so much that Kathy Bates wasn't cast, but that Michelle Pfeiffer was. Pfeiffer is stunningly beautiful and not at all the Frankie type. The producers didn't trust the material so they went for box office stars. But there were several actresses that were more appropriate for the role.
Garry Marshall directed it and I always thought he should have persuaded his sister Penny Marshall to play Frankie. Diane Keaton could have played it. Lily Tomlin could have played it.
If anyone ever tells you that you put too much Parmesan cheese on your pasta, stop talking to them. You don't need that kind of negativity in your life.