This is due to the refractory period at the end of each impulse. From here
The cell membrane cannot immediately produce a second AP. The period of time when the majority of voltage-gated Na+ channels are inactivated defines the ABSOLUTE REFRACTORY PERIOD, when no amount of depolarizing current can cause an action potential.
This refractory period is due to the gating mechanics of the voltage-gated sodium channels: From here
Refractory periods are caused by the inactivation gate of the Na+ channel. Once inactivated, the Na+ channel cannot respond to another stimulus until the gates are reset
Because of the way impulse are propogated, this same principle explains why impulses are unidirectional under normal physiological circumstances. Again from BYU:
an action potential might have started near the cell soma and as it propagates down the axon towards the opposite end, the membrane potential behind the moving action potential has repolarized and returned to resting membrane potential. The axon ahead of the current depolarization has not yet depolarized and it is also at resting membrane potential. Where the action potential is occurring we find the membrane potential depolarized and the outside of the membrane at that spot is negatively charged relative to the inside of the membrane at that spot. As sodium rushes in, it will depolarize the next adjacent spot on the axon in the direction that the action potential is propagating. The reason that the action potential does not depolarize the section of axon membrane behind (or in the direction that the action potential just came from) is because that section of membrane is most likely in refractory periods and does not depolarize.
To demonstrate this, one could actually artificially trigger an action potential in the center of an axon (normally impossible) and observe it travel bidirectionally.