Its probably expensive. I know CF in tripods and I know when its cold like -20C it can shatter so I guess there maybe problems when temperature changes. I know Kevlar is used in some speakers now so maybe CF will become more common.
Here is something about CF: "The next generation or rigid-cone drivers were the Japanese carbon-fiber units, which made their first appearance in the pro studio monitor (prosound) 12" TAD units with very high efficiencies and very high prices (around $300 each in 1980). Carbon fiber prices have now dropped, and Vifa, Audax, and Scan-Speak make good examples of this type of driver. The Japanese make lots more of them, having pioneered the technology, but they are very difficult to obtain if you are a non-Japanese small-run specialist manufacturer.
These drivers have true piston action, outstanding bass and midbass response (the best I have ever heard), but also have a characteristic double-peak region at the top of the working range. Unfortunately, these peaks are grossly audible in most carbon-fiber drivers, and worse, cannot be removed by a notch filter tuned between the two peaks; it requires two notch filters to control the peaks, or a low crossover with a very sharp rolloff (24dB/octave) to remove them from audibility.
Although I very much dislike drivers that require filters as complex as this (after doing the TLM-200, I vowed never again to design a 57-component crossover), I must admit that carbon fiber woofers are the only direct radiators where I've actually felt tactile bass."