(Weak) convention? Most US cars have the gas cap on the left side but there's no inherent design reason even if some people have preferences and it's by no means universal.
The driver sits on the left in the U.S. And, lacking widespread public transit, most driving hours are commuting... alone.
Having the gas cap on the left both makes it easier to pull up to the fueling station without being too far or bumping into it, and - having done so - prevents the driver from having to walk around the vehicle.
That is one point of view. I've also read others who say that they prefer the right because it means that there isn't a gas pump right next to their car door--which means they have to carefully open their door so they don't ding it.
I probably prefer left but don't have a strong opinion.
And still another that says it should be on the right because if you run out of fuel you are away from other traffic while refueling.
In truth cars are designed for the entire world. Designers will switch the steering wheel side for the country, but they will avoid having to make changes to something far away there that doesn't need to change.
The bottom line is that there are weak preferences, weak arguments for right vs. left (or clockwise vs. counterclockwise), and generally weak design asymmetries/constraints for a given situation and you end up with a majority (but by no means universal) choice.
Not saying that's the case related to the OP, but it seems reasonable absent specific evidence to the contrary.
