The reason why most are on the left side is that the majority of the world uses LHD cars. Only UK, Japan, and a few others are RHD. With the filler on the left, it is easier to fill (don't have to walk around the car).
I believe it used to be that the US cars were typically filled on the right side and other cars were mostly filled on the left. Now, it really is a mix with multiple cars sharing a single platform and many foreign cars being built in the US.