The story is mainly about friendship, if the parents were brought in, then it would pull away from the main plot. There is the slight idea that, Sora might have understood there was no saving his parents, but his friends he COULD do something about and went with it. Also, he was so thrust into the thick of things, it's hard to think of your parents. Last, it would meen they died. Yes, died. Disney doesnt have much liking towards death here people, why do yoiu think ansem keaps creaping back? lol.