The 1952 René Herse featured in Rivendell Reader No. 26 (shameless plug - I wrote the story) has under BB cable routing. So Sheldon is right - it's an mtb thing (the Herse has 650B wheels), but not an American mtb thing.
How about this postulation: Clamp-on guides usually were above the BB - whether because it was easier to make them that way, or whether because nobody thought about it differently, or whether there was some long-forgotten (by me, at least) derailleur that needed above-BB routing. So when the braze-ons came along for ITALIAN/AMERICAN (the two really were more or less the same at the time) bikes in the 1970s, they copied the clamps. That could be all there is to it.
Whether the below BB routing came about because somebody finally saw a René Herse (or Singer, they used the same arrangement), whether it was U.S. mtbs (as Sheldon postulates), or simply that somebody's clamp broke and they had to cobble together something and figured that the cable could run on the BB shell just fine to get them home...
Now, who invented that stupid idea of having a piece of cable housing for the front derailleur cable near the BB, where the water collects, rust develops and the front derailleur stops shifting? My 1965 Cinelli Supercorsa has that one, without the rust, fortunately.
Jan Heine, Seattle
P.S.: Somebody please explain to me how the brake cable routing inside the top tube came about in the early 1980s (?). Hint: The 1952 Herse has that feature as well.