We are making assumptions about under BB cable guides being OLD. I've seen ample examples of Constructeur bikes with guides mounted beneath - dating from the 1940s & 50s.
With bikes built for dedicated derailleur models (with braze-on hangers in front of the rear dropout) it sometimes made more sense to route the wiring beneath. That said... there were also a great many production bikes from the 1950s through 60s and even later with "pairs" of simple rear cable stops brazed onto the down tube and chainstay for the rear derailleur .
And then when cable operated front derailleurs came into common usage, a single stop on the left side of the DT for a front derailleur was added. (Think of a common Peugeot UO-8, well into the 70s). These all required short lengths of cable housings which route the rear cables around the BB shell. These make the most sense to me, but these too were really designed with certain common derailleurs of the era in mind (e.g.: Campy GS or early Record or Simplex Prestige through early Super LJ, etc. front derailleurs - all with integrated cable housing stops). The use of the enclosed housings over the most vulnerable (dirt & rust accumulating) BB region worked well and in fact would be even better using modern "compressionless" cable housings.
But I agree, the trend toward adding simple below the BB guide plates was just a cheaper solution than any other style. And, especially with typical modern dropped-parallelogram rear derailleurs, running cables below the chainstay is no problem.
Personally, I'm just happy to see ANY guides on a bike - rather than clamp-on dirt & rust collectors.
BOB HANSON, ALBUQUERQUE, NEW MEXICO, USA
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