Presumably the preference is for natural fibers over polyester.
You can't just wave a magic wand and say, "Let's use natural fibers!". What natural fibers, exactly? Cotton? Bamboo? Rayon? Merino Wool? These all have different characteristics than nylon, polyester, or polypropylene that make them less suitable for use in outdoor clothing and gear. I've seen absolutely zero natural fibers than can compare to 15 denier coated polyester for making ultralight tarps, or to ECOPAK UltraWeave for making lightweight backpacks.
The same goes for synthetic insulation (Primaloft, Climashield Apex, Polartec PowerFill, Thinsulate) vs the natural stuff (i.e. down). They've tried to make down perform better in challenging, damp conditions by spraying a bunch of DWR chemicals on it. It only works to a point, and then the DWR gives up and the down will still collapse and clump.