I did a Google search to see if I could find a reason. . .I did find a few debates on this topic.
Some (such as MBB and myself) feel that the door should be on the driver's side.
It is aggravating to always having to walk around the car to get gas.
Others think it should be on the passenger's side so that the driver doesn't
have "space issues" with hitting their car door on the pump (I don't get this
one. . .just pull forward a little bit more!) or so that the driver doesn't get hit when they have run out of gas on the side of the road and need to fill-up from a can (duh - don't run out of gas!). The only thing that I did find is that the gas tank should be on the opposite side of the muffler.
I found another neat tidbit (No, I haven't researched this, it just sounded neat!):
You can tell what side of the car the gas door opening is by looking at your gasThings that make you go "Hmmm..."!
gauge. Some automakers include an arrow next to the fuel pump icon to tell you which side of the car needs to be next to the gas pump.