It’s my understanding it’s due to the anatomy of a peripheral nerve. To keep this simple, the anatomy of sensory v motor fibers are akin to a coaxial cable. The sensory fibers are the outside of the nerve(insulative portion of the cable) and the motor fibers are the inside(copper wire).

Thus when compressing the nerve the outer layers are effected first and the inner fibers require more compression and time to be effected compared to the outer. This leads to sensory deficits superseding motor deficits.