It mostly has to do with the exhaust pipe. And the routing of the pipe and muffler depends on the engine, where is a good place to get past the transaxle and bring the pipe back, etc.
Right. Because the pedals, brake lines and throttle cable are on the left, you don't want to run the exhaust too close to them and risk the heat from the catalytic converter melting or boiling any of those parts (or the driver's feet). So, the exhaust almost ALWAYS ends up on the right, on cars for US sales, at least. So, if you already have a bunch of sensitive cables and brake lines running up the left underbody, add a fuel line, and maybe a mechanical gas door release cable, too. Now, it makes perfect sense to put the gas tank on the left, and the muffler on the right - or the muffler can cross over behind the gas tank.
That only makes sense with completely separate dual exhaust systems, or with the muffler in front of the gas tank, and the tank RIGHT ahead of the rear bumper. Hmmm, not the best place to put a crush hazard, is it?
Jon