A couple of different sources I've read lead me to chalk up any popularity of the CC and PR shifting systems to unreasoning bias, and rabid nationalism.
There was this thing in Italy about the idler wheels in conventional derailleurs: supposedly running the chain through the wheels caused friction which could slow you down over a long stage, compared to the relatively frictionless CC or PR shifting system. I'd love to see some kind of objective data on this, but I don't know of any.
Also, I believe it was the case that the conventional derailleurs used by most people in the 30s and 40s were french, and there was a bias against using anything that wasn't italian--an even bigger problem for italian racers was that *tourists* used idler-wheel derailleurs...any self-respecting racer wouldn't be caught dead using something a *tourist* used...
Note however, that the moment an *italian* company made an idler-wheel derailleur, the CC and PR systems were dead. Campagnolo killed off their own products...of course, the GS der was a distinct improvement... and it was marketed as a racing product, which helped.
All this said, I have to agree with Chuck: there is something fine about riding along with no idler-wheels interfering with the chain and causing all that nasty friction.. <g>
Charles "had a Bianchi PR once--I always worried about bending one of the drop-out teeth" Andrews SoCal