Question

Why is transmission between neurons unidirectional?

Answer 1

Because of the chemical nature of these transmissions and the structure of the neuron

Explanation:

I invite you to look at SCooke's answer to this question for some detail.

Basically, neurons are shaped with a cell body as the control centre, up to thousands of dendrites recieving information, one axon transmitting information, and one axon terminal which allows for said information to be transmitted. Because action potentials can only travel from dendrites through to the axon, the transmission must be unidirectional.
https://biology.stackexchange.com/questions/21986/are-neural-connections-one-way

https://en.wikipedia.org/wiki/Neurotransmission#General_description

http://www.mind.ilstu.edu/curriculum/neurons_intro/neurons_intro.php

I hope I helped!