Why does oxide basicity generally increase down the alkaline earth metal group?

The basicity of alkaline earth metal oxides increases down the group (from BeO to BaO) primarily due to the increasing metallic character and the decreasing ionization energy of the elements as you move down the periodic table.12

As the atomic size of the alkaline earth metal increases down the group, the valence electrons are held less tightly by the nucleus, making the metal more electropositive. This trend facilitates the formation of ionic bonds between the metal and oxygen. Because these oxides are ionic, they react with water to form metal hydroxides, which are basic. The increased ionic character of the metal-oxygen bond in the heavier members of the group enhances the compound's ability to act as a base, as the oxide ion ($O^{2-}$) is more readily available to react with acids or water.321

In contrast, smaller elements like beryllium have a high charge-to-size ratio, which leads to significant covalent character in their bonds, resulting in lower basicity compared to the larger, more electropositive elements like barium.13