The reason is a deeply complex anatomical wound that every playable character has in the thighs (which also explains the reason why buttocks are not naturally shaped). The doctors said that we would never be able to run again... But! With enough willpower, we were able to overcome this weakness and still managed to find a way to run anyway!

Unfortunately, we had to sacrifice our ability to run backward. A shame, really... We should all be angry toward Bahamut, to inflict such a terrible wound to all of us, and kill him! Then, we need to roast him and eat him. Only then, perhaps, we could recover and finally gain the ability to run backward!