BECAUSE YOU ARE RECORDING ONTO A STEREO TRACK!!!
You are recording through a regular standard microphone (or whatever other mono source you are trying to record)...
A stereo track is made up of a "LEFT" and "RIGHT" component.
You are recording onto the "left" or "right" side of your stereo track and leaving the opposite side empty...
This is causing you hear sound from only one speaker.
Because you are only recording to one side of the stereo channel.
Unless you are recording something that has 2 individual outputs that you plug into the "left" and "right" inputs of your interface -- YOU SHOULD BE RECORDING ON A MONO TRACK!
That is the reason you are only hearing sound from one side.
...and, yes, that is ALWAYS the reason
(when all else is working as normal and unless you have your pan knob off center, or you have a plugin on your track that is forcing it to one side, etc.)
...unless it is something else