Why alkaline earth metals are harder than alkali metals?
More electrons, more positively charged nuclei......and thus greater opportunity for metallic bonding.
As you know, metallic bonding results from the delocalization of electrons from the parent metal atom so that positive ions are conceived to swim in a sea of electrons. This view of metallic bonding accounts for the malleability, and ductility of metals, as well as their properties of thermal and electrical conduction.
The alkaline earths have greater nuclear charge, and more valence electrons....and thus metallic bonding should be more effective.