I'd speculate that ("bad batches" aside...) when the point came that lawmakers enacted a youth requirement on tires via "fix-it tickets" from annual state inspections or spot check, tire manufacturers were freed from obligations to make tires which might last longer than six years. It would be silly to do so from a business standpoint. Quality has been adjusted accordingly. Other factors surely apply but this seemed to happen around the time that 15"s were no longer kept in dealer stock and tire names we used all our lives were replaced by a random stream of cut rate tire brands. Meanwhile, annual state vehicle inspections here were dropped in favor of allowing traffic stops as vehicle inspections.
So, law change is my guess. Crummy quality tires are the standard now and anything small is niche and worse. The average no-name popular general public size tire is fairly round these days so its not like they forgot how but focus has shifted. My daily drivers wear 16s... same thing now. Its up to 17s for modern brakes and fresh decent tires. Thats my take anyhow. I ain't cleaned a whitewall this century.
So, law change is my guess. Crummy quality tires are the standard now and anything small is niche and worse. The average no-name popular general public size tire is fairly round these days so its not like they forgot how but focus has shifted. My daily drivers wear 16s... same thing now. Its up to 17s for modern brakes and fresh decent tires. Thats my take anyhow. I ain't cleaned a whitewall this century.