Porcelain is a non-porous material that can withstand fluctuations in temperature.
Porcelain is also very smooth, and thus easy to clean.
Millions of gallons of drinkable water are flushed down North American toilets every day. This, at a time when drought and desertification prohibit many people from accessing adequate sources of clean drinking water. It demonstrates that the toilet is not an environmentally friendly object. It wastes a lot of valuable water, and by flushing wastes into oceans, etc. If steel had been used, the bowl might rust, and the seat would be very cold.