Mainly because the prices of gold have skyrocketed, being one of the most favored tooth filling materials around. Gold is one of the best materials to use for dental fillings due to it’s inert qualities and it is way less toxic than dental amalgam which is loaded with mercury. Composites or materials that are combinations of several non-metallic substances(an example would be the 5-minute epoxy you use in the house) to form a strong material.
Ceramics and porcelains are considered to be the cheapest substitute to the precious metal which may have lost ground but may not totally disappear from the dental restoration scene. Gold has a better appearance(even if it’s merely a thin sliver bonded to a ceramic base) which still makes it the most preferred material. But ceramics are being prescribed more openly because of the price of gold which has increased as much as 105% these past few years.