Why solubility of sulphate decrease in group 2

This had to do with the resulting enthalpy changes of the sulfates.

\Delta H_{solution} = \Delta H_{hydration} - \Delta H_{lattice}

If delta H solution is negative, it means the solid will readily dissolve (the more exothermic a reaction the more feasible it is).

Well, the delta H lattice is always negative and it's magnitude decreases down the group.

Delta H hydration is always negative too, because bonds are formed between the gaseous ions and water molecules. It's magnitude also decreases down the group as the charge density decreases down the group.

What happens, is that the delta H hydration becomes positive 'faster' down the group than the delta H lattice. As a result, the delta H hydration becomes more positive. And as delta H becomes positive, the reaction becomes more endothermic and less feasible.