Ethel Merman and Mary Martin were a generation older than Sondheim, and he did not represent the Broadway they knew.
Merman requested that Sondheim only write the lyrics for Gypsy. Years later Sondheim wrote special lyrics for "Send in the Clowns" for the one-night concert on Broadway by Merman and Martin. Ethel and Mary opened the concert by singing "Send in the Clown" backstage and then on stage.
The producer of the concert had to find someone in the audience who did a bootleg to fill in words not picked up backstage.