According to Drago's rule lone pair on phosphorus resides in almost pure s-orbital, hence due to non-directionalnature, its overlapping tendency is greatly reduced in comparison to a lone pair present in hybrid orbital, which is directional as present in $\overset{\,\,\centerdot \,\,\centerdot \,\,\,\,\,\,\,\,}{\mathop{\text{N}{{\text{H}}_{3}}}}\,$.