Stability of +5 oxidation state decrease from N to Bi. Why?

The inert pair effect relates to the increasing stability of an oxidation state that is $I I$ LOWER than the Group number, and is observed in Groups 13, 14, 15, and 16, for the heavier Group members. Nitrogen and phosphorus, as Group 15 elements, can be oxidized up to $N \left(V +\right)$ to $P \left(V +\right)$, the Group oxidation states; lower elements of Group 15, TEND to display stable $+ I I I$ oxidation states, (the which is certainly true of phosphorus).