Because porcelain is probably cheaper and easier to manufacture (mold) and clean.
It also doesn't deteriorate (or corrode) like plastic, wood, and metal and it adjusts better to ass temperature.
What the heck do I know?
Ask the Mancesa people.
As a side point, I once replaced one of my wooden toilet seats with a plastic version, and I felt like a I was riding a trolley, it shifted so much.