As interesting as the idea is, I don't think the analogy with Darwinian evolution quite holds. In order for something to act as a selective pressure it needs to actually exert pressure. That is, just simply saying, "Well, if it weren't that way, then certainly someone would jump in with an alternative" doesn't work as an explanation because it's never happened. Taking it back to biology, it's as if someone asked "Why don't we have 3 eyes?" You could come up with all sorts of reasons why 3 eyes might be worse than 2, but until someone is actually born with 3 eyes and has trouble surviving because of it, none of your reasons would be valid selective pressures.
Edit: As to the original question, I imagine the right answer is something more along the lines of: where to place the fuel door is considered rather unimportant, and so left until last. At that point, it goes wherever it will fit...