Action potentials are generated at the axon hillock of a neuron and travel in one direction towards the synaptic terminals. Why are action potentials only transmitted in one direction under normal conditions

The sodium channel have refractory period.

Explanation:

Action potential are generated at axon hillock of neuron and travel across the neuron in response to stimuli. For the the generation of action potential the steps are stimulus, depolarization, repolarization, hyperpolarization and resting state.

So the first step for action potential after stimulus is depolarization. Depolarization requires the opening of sodium channels to make the inside of neuronal membrane positive. These sodium channels have refractory period. They cannot open immediately after activation. So the sodium channels of one direction in neuron (upstream) become refractory and it is convenent to open the sodium channels of other direction (downstream) of neuron. That why action potential is transmitted in one direction.

It is like the gunpowder placed at a line the fire cannot go back in burnt gun powder direction.