It is part of the colligative (collective) properties of all solutions. 

Suppose a little molten metal X is added to molten metal Y to form an alloy with metal Y being the solvent and metal X being the solute. 

In most cases, only the solvent can form the solid as the solute would not perfectly fit into the solvent's solid structure. 

In very simple terms, not only do you need to remove energy from the atoms to force them close enough for them to form the solid, you also need to use energy to separate the atoms of X and Y in order to allow Y to freeze. (Imagine trying to separate a needle from a haystack) Thus, the melting point decreases as you need to remove more energy than usual.