Going down Group I, the number of electronic shells increases, and atomic radius increases. As the valence electrons get further from the nucleus, the attractive force between the positive protons in the nucleus and the negative valence electrons decreases.

Hence, going down Group I, the metallic bonding formed between the positive protons in the nucleus and the delocalized valence electrons becomes weaker.

The weaker the metallic bond, the lesser the energy needed to break these bonds, and the lower the melting point and boiling point of these elements.