Five year old article on the "15 inch tire problem"
Tires are made in production runs. Only so much time to make so many tires in any size. Only so much production space/tooling.
Modern wheels/disc brakes won't clear a 15" tire/wheel. So demand for 15" tires in "our" sizes is in the thousands annually. Demand in other sizes in in the millions.
Not rocket science. That is the reason that companies like Coker purchase old tire molds and contract to small companies who rely on short runs of specialty tires. Also why the unit price is high. Always will be.