Comment by giraffe_lady

I'm not completely sure and I only have the insight of an interested hobbyist here but I don't think it's a single reason.

A lot of it is that the production improvements have mostly been in developing processes for synthetic replacements. Natural fibers are agricultural products: wool comes from sheep, that are raised on land, harvested and processed by skilled laborers, with natural variance in the input & output; linen, cotton, silk different variants of the same constraints. Polyester is not like this and it indeed can scale vastly and be very cheap. Rayon can be produced from basically any cellulose input so same.

So a lot of what would be productive gains have just shifted over to these other modes. Cotton is the main natural fiber the industry focuses on and it is mostly a lot cheaper now than it was in the past.