Because #Ar# has a larger, more diffuse electron cloud than does #Ne#.
Explanation:
For #Ne#, #Z=10#, for #Ar#, #Z=18#. Thus argon has 8 extra electrons in a more diffuse electron cloud. The electron cloud of argon is thus more larger and more polarizable, and more able to express intermolecular dispersion forces. The result? The #"boiling point"# of #Ar=87.2K#; of #Ne=24.5K#.