I don't think that the fabrics were especially wrinkle prone, but they were, in the case of the notorious polyester double knits, fabrics that  needed warm temps to relax the wrinkles, but then needed to be cooled before the spin to prevent new wrinkles from being set into the fabrics IF A DRYER WAS NOT USED TO FINISH THE LAUNDERING PROCESS. The wash and wear and permanent press laundering process needed heat to relax the fabric fibers to eliminate wrinkles. If a dryer was used to supply heat to the fabrics, and the cycle ended with a cooldown, the washing process was not as important.  Knits could then either be treated as delicate fabrics or regular fabrics and washed on cycles without the post wash cooldown.

When I first was working up here, I had a colleague who was living in an English basement apartment with a WP portable set. She mentioned that her clothes were not shedding their wrinkles in the laundering process. She was from California and just out of graduate school and very environmental. I asked how she was handling them. She was washing everything in cold water in the winter in Washington DC where the tap cold temperature was in the 40s. She was drying in a 120 volt dryer that was not getting the fabrics warm enough to dewrinkle in the dryer.  I explained what I posted above and her results improved. 

The knits and permanent press cycles were designed to give the best performance possible from the washer even if no dryer were used.