First, you only see the quilts now that DID last! All the ones made badly or from bad fabric are long gone.
I think the quilts that lasted were made from great fabric. And my sense is, the base level of fabric available to them was better than some of the really shoddy goods we find now...why? I think it was much harder in that smaller, more personal world to remain anonymous as a manufacturer, and I think your average housewife was quite knowledgeable about fabric and fabric quality, since most people made at least some of their own clothes and household goods.
Now, any shyster can get a run done in China and flog it to the bargain wholesalers. There is much less feedback from the end consumer.
I did have a very pretty quilt given to me when I was in my 20's, made by a local lady. My mom bought it for my birthday, and the thing totally self destructed. The cottons used were cheap, very low density and they frayed and fell apart. It was the classic example of poor materials and probably poor workmanship as well.