In the US, there are no regulations that specify where a fuel door should be placed. So, car company engineers can place the door to the gas tank on whichever side offers the easiest packaging. The placement of the fuel door is mainly a factor of fuel tank design, location, and underbody packaging. With all of the structure and components located underneath the vehicle, engineers would quickly encounter restrictions in trying to route the filler tube to the same side on every vehicle. If mechanisms such as a big speaker must be placed on the left side, engineers put the fuel door on the right. Fuel door positions is not a random choice, but if engineers have a good reason to place fuel doors on the right, that's where they go.