Probably because it’s too difficult to produce in quantity.🤔
And boron arsenide
Boron - need mining See Borax mines & found in volcanic spring waters.
Arsenide - arsenic - so toxic! So, a potential environmental problem.
Enviro harpies won’t like that!
Key problem: from article
“.... practical ways of making this material in usable quantities. The current methods of making it produce very nonuniform material ....”