Since the atoms of alkaline earth metals are smaller in size and have one  valance electron more than the alkali metals of group IA,therefore they are more closely packed and less tightly held. Hence the inner atomic attraction in the alkaline earth metals is greater and they are harder than alkali metals.

As we go down the group the atomic size increases and the atoms are packed less and closely and held less and less tightly.Thus the atomic attraction decreases and hardness decreases.

 Oxides of group 2 were originally thought to be elements, not compounds.  These "elements" were called alkaline earths: "alkaline" because they're basic when added to water, and 'earth' because that's what they called oxides that they thought were elements.

The Group IA elements are called alkali because when they reacts with water they forms basic solutions like Na metal reacts with water and form NaOH a strong base.