Mercury atoms have two outer valence electrons which are quite tightly bound to the atom thus preventing the formation of strong interatomic bonds. Although Mercury is in the Transition Block of the Periodic Table, it is not treated as a Transition Metal since its chemistry never involves an unfilled d-orbital.
The elements Thallium, Lead, Bismuth, Tin, Indium and Cadmium, found near Mercury in the Periodic Table, have quite low melting points also, for similar reasons.