Ok, just stop and think about it for a minute.
Done? Right: the musical theatre literature prior to the 1960’s was based primarily in a “legit” style, unless the character in the musical was there for comic relief, or was an older, light weight character.
For example, Ali Hakim in Oklahoma or Nettie in Carousel or Meg in Brigadoon. A ballad is not typically the style these kinds of characters sing in (ok, of course Nettie sings “You’ll Never Walk Alone”, but she’s an usual case). So if there are only a half dozen men’s ballads pre-1965 in The Belt Book, it’s not because we haven’t done our homework, it’s because that’s not how it worked in the Golden Era of the American Musical Theatre.
Have suggestions? Let me know!