Down the group `(darr)` i.e. from `S` to `Po`, tendency to show `-2` oxidation state diminishes. Why ?

The tendency of showing-2 oxidation state diminishes from S to Po due to

Why Eu and Yb shows +2 oxidation state ?

Why does zinc show only + 2 oxidation state ?

Account for the following: Tendency to show -2 oxidation state diminishes from sulfur to polonium in group 16.

Among the elements of group 14 the one having highest tendency to show stable +2 oxidation state is