I dont know that cotton was ever all that expensive. i think it was more the general zeitgeist-space age, march of progress, man-made high-tech to match. i remember reading a hand-me-down early 60s novel as a child where the protagonist marvels at how quickly his shirt, made of "these new synthetic materials", dries after he gets soaked in some pratfall. ads in magazines describe things like dacron as nothing short of miracles, the 60s equivalent of penicillin or something. synthetics were also fairly liberating to 50s housewives. the idea that pure cotton is any more comfortable or better looking isnt really considered by many people even now, particularly women who do a lot of laundry.  back then the vast bulk of the population had grown up wearing coarse, itchy wool clothes and even underwear.
another thing to remember is the social revolution that occurred here post-wwII. the people filling the new white collar occupations created by postwar growth, educated most likely on the GI bill , were literally a half-generation out of the ditch or the factory or the farm. these people wanted to look as neat and tidy as possible. the idea that something was better if it was slightly seedy/passed down from earlier generations would be completely alien to them. most of these people had grown up wearing hand-me-downs during the depression, and it wasnt a mark of "sprezzatura" to them. wrinkled, frayed, comfortable clothes were the indication of a hobo.