Why is a chemical synaptic transmission unidirectional?
The synaptic transmission carrying instructions from the brain has to travel towards the site of action.
The brain communicates with the body via synaptic transmission (chemical and electrical, usually coupled together).
This means the signal has to reach the body part that the brain wants to control/move specifically.
Synaptic transmission involves information (brain's instructions) travelling (transmitting) along from axon to axon (an axon synapses with another axon) from the brain towards the site of action.
Using an analogy, an axon is a pipe and the information is the water flowing through it (flow of water is unidirectional only). In order to reach its target cell, information can only flow in one direction (unidirectional) - away from the brain and towards its destination in the body to carry out the instructions.