But that might be changing. Plastics Today is reporting of an attempt by FRX Polymers to commercialize a polyphosphonate polymer for use as a flame retardant. There are both homo- and copolymers (carbonate being the comonomer), they resins are clear and they mix well with a variety of polymers. Being non-brominated is a big plus for their use as flame retardants as there is ever increasing pressure to move away from those materials.
Hopefully this product can make it. It would have only taken about 60 years to go from the first discovery of polymers with phosphorus to the first commercialization (at least no one as been bankrolling the venture for all that time), but it might just be worth all the wait.
Update: Thanks to Neil Withers for pointing out my misspelling of the 15th element. It has been corrected. (At least I consistently misspelled it!)