Here's why:

* Momentum is a measure of an object's mass in motion. It's calculated by multiplying an object's mass (in kilograms) by its velocity (in meters per second).

* Newtons (N) are the units for force, which is a different concept than momentum.

* Kilograms per meter second (kg/m s) would represent a rate of change of mass, which is not directly related to momentum.

Therefore, kg m/s is the standard and accurate unit for momentum.