Sometime Microfiber, which is polyester, polyamide and/or polypropylene, or pretty much plastic, is being used for inexpensive bedding including pillowcases. If you have ever tried to cut and sew microfiber you will have seen how easily It falls apart, threads pull out, and doesn't drape well as it is too light weight.
Medium weight cotton not only drapes more elegantly it also holds up to many more washing and use. As pillowcases can get a lot of use, often falling apart before the sheet set they are sold with, we suggest trying a medium weight or quilting weight fabric for pillowcase construction.
Weather you make your own or choose which ones to buy, look for soft 100% medium weight cotton. You will be glad you did!