Why Avogadro’s theory is called a hypothesis even though it is proven and become theory?

Amedeo Avogadro’s hypothesis is also called Avogadro’s hypothesis despite being a proven theory. There is no logical reason behind it. While Avogadro could not prove his hypothesis theoretically, it was not fully accepted but could be used to arrive at the correct solution. At that time this unproven hypothesis was widely discussed as Avogadro’s hypothesis. It was later proved by Millikan, but its hypothesis remains. But you can call it theory or formula if you want. Rather that is more acceptable.