Well, CHEAP certainly leads the list, in my opinion. But LIGHT also points to the logic, surrounding the issue. Sadly, plastic is not as strong as steel nor, apparently, as resistant to the passing of time(that last comment is my opinion only). However, size for size, it was lighter back then.

Dupont's Delrin plastic, used in some Simplex products, is a prime example of a good, but poorly engineered, idea. Perhaps the derailleur, presented by the OP will suffer from the same, or similar, woes, as did the Simplex chain jumpers, which brings to mind a question that is all but guaranteed to start up flames all over the place.

Just exactly what is Carbon Fibre? Some sort of magical carbon stuff, held together in a plastic resin. So, if we look at today, and think back fifty years or what ever, Delrin plastic might well have been the wonder stuff of its time, not just a cheap alternative to steel, or brass or aluminum.

Finally, although I am not a fan of the plastic used by Simplex, I am a fan of the transmission, itself. They offer very smooth shifts and I like them, when they are not broken in half...