Because it doesn't offer any obvious (even if they are insignificant) rider advantages.

Making a bike lighter(even if it's by an entirely insignificant amount) will always be a good selling point.
Because regardless of how marginal, making a bike lighter is still a move in the right direction(unless, perhaps if you have to be UCI-compliant).
Making a bike more aero is also a move in the right direction(unless you collide with UCI rules again).
Claiming that CF bars/forks/stems/seat posts gets rid of road buzz, well, there's an easily understandable improvement there too, whether it's measurable or not.

Now when you mention them, I'd rather like to have CF brake levers on my winter bike. They wouldn't sap as much warmth from my fingers as the alloy levers do.

But if the current standard is good enough(ie, rarely fails), there's no advantage to the rider by improving it - unless with something that's lighter, more aero, stiffer, comfortable etc etc.

If you'd want to market it, you need to tap into something that would be perceived as a ride/performance improvement.

Maybe claiming that it makes the crankset stiffer would work.
When people started transitioning from square taper to the various versions of hollow axles and external bearings and all that, crank stiffness was a frequently quoted advantage. As precession is due to the pedal axle wobbling around a bit, so maybe one could claim that having a pedal axle that remains a little more perpendicular to the crank would make pedalling more efficient. There's a potential selling point.