Au contraire, Robert: I found it in 4 WWII songbooks (though only two were period). Which doesn't change your overall point, being that you'd expect it to be more prevalent given the wide post-war recording popularity.

Of course, it is also referenced in newspapers as being sung some. The first reference I found was in December 1943, when one columnist relayed that "The evening's favorite [song] was that famous carol, 'I Used to Work in Chicago'" (The Slip Stream [Supplement of the Casper Herald-Tribune], December 29th, 1943, p. 5). Columnist Harold Martin described in April 1945 an air force recruit who could sing 2,000 verses of the song (The Atlanta Constitution), and it was also sung by post-war occupation forces (The Bradley Tech, November 15th, 1945, p. 5). Not a great deal of references, but it was certainly sung.

Perhaps the explanation is that it was sung *some* during the war, then the Three Bits of Rhythm sang it and it sold well, and others rushed to cover it. Or perhaps it jogged enough of a memory to bring something to the mind's front.


Songbooks:
- Songs of the Century (1945) [Songbook of the 100th Bomb Group]
- Song Book of the Wild Hares
- Bawdy Ballads and Dirty Ditties of the War Time RAF
- Army Air Force lyrics : a collection of WW II U.S. Army Air Force marching songs, poems, and parodies to popular songs of the period and the past (reference only)