Honestly, I do believe there was so much polyester back in the '70's that everyone used it. You really had to hunt for something other than calico cotton. I remember when I first started quilting in 1978 I would go to Woolworth's and TG&Y for cotton fabric. House of Fabrics had a lot of fabric but it was expensive. Sears had some fabric and yarn back then but it was mostly polyester and most quilt instructions told you to use cotton. I did buy the polyester batting because it seemed to hold up better. Also, it was known the polyester fabrics held their color better than cotton.
Personally I preferred to sew with cotton or wool. It didn't slide as much.