I think they want to leave it exclusive for the larger cars. I remember back then when my Goodyear kakis were around they mentioned the F1 GSD3 had 15" upwards, and they had the most complaints from people of 15", that is why when the Asy started, it only comes in from 17". Likewise PS2 those days only from 17" up, then PS3 had 15", that is why PS4 now also no more ......
Don't want, too thin, harsh, and expensive. Only my Mark X uses 18 and for that car I am just gonna get S1E3 later when comes time to change. Anyway if 18' I might as well get F1SS if I want to.