Why did the early metal oxide FETs use metal as the gate stage?

It was due to the advancement of semiconductor technology that the metal oxide field effect transistor (FET), which used metal as the gate level, was replaced by polysilicon. Gold-oxide half-field-effect transistor in the structure of a metal - oxide layer - semiconductor capacitance as the core (now the gold-oxide half-field-effect transistor is mostly polycrystalline silicon instead of metal as its gate material), the material of the oxide layer is mostly silicon dioxide, under which is the silicon as a base, and on which is the polycrystalline silicon as a gate.