Why doesnt blue cross cover braces for autoimmune patients?

My daughter was diagnosed with a rare autoimmune disorder called Autoimmune encephalitis which causes her great distress and we are now on our 3rd round of antibiotics trying to find the right one. With that being said she also needs braces but since she can talk and chew they are not considered medically nessacary. I then wrote an appeal that included documentation and doctor findings, literature and whatnot. I just opened the mail today and got a denial letter. I am extremely heartbroken considering the money we have spent this year going to so many doctors and money and treatment. If her teeth have any type of overcrowding, they will then have to administer prophylactic antibiotics so that her immune system does not start attacking itself and the brain. I'm in tears costs are through the roof and you guys seem to love to milk the insured.