I read somewhere that the problem is two-fold: the design department's hesitation to use any sort of strain relief and the softer type of coating that is supposedly more eco-friendly.
Eco-friendly becomes something of a joke if you need to buy a significant extra number of them...
It might be simpler than even that. Most cables are black because black ones are more durable for many reasons(UV resist and more choice of composition/processing)
Apple likes their bright colors too much to do that :)