The melting point of calcium is higher than potassium because of its stronger metallic bonding. Calcium has a smaller atomic radius and a higher ionization energy than potassium, which means that its valence electrons are more strongly attracted to the nucleus. This results in a stronger metallic bond and a higher melting point.

In addition, calcium has a face-centered cubic crystal structure, while potassium has a body-centered cubic crystal structure. The face-centered cubic structure is more closely packed, which means that there are more atoms in contact with each other and more metallic bonds holding the structure together. This also contributes to the higher melting point of calcium.