Initiation of nerve impulse occurs at the axon hillock because:

The membrane of the axon hillock/initial segment contains a high density of voltage-gated Na+ and K+ channels. These channels are fewer along the dendrites and cell body. The axon hillock has a lower threshold for firing an action potential. This region is also known as the trigger zone for the action potential. It requires a relatively low degree of depolarization for initiation of spike at the axon hillock/initial segment.