As the father of an elementary-age boy, I think I know why God places so much emphasis on obedience.
Boys can get themselves into so much difficulty so easily.
I can't imagine the trouble they would get into if they weren't required to obey anyone. Take, for instance, my rule about crossing the street. I expect 100-percent obedience because I know that it takes only one careless step into the street to jeopardize his safety. When he didn't, Saul not only failed Israel but he disappointed God. And he had to suffer the results of disobedience.