Since your butt is also proximal to the end of the digestive tract which is the anus, this means that your butt is near the last stop of the smelly fecal matter. What makes the feces smell are the dead pathogens and waste products processed by the body.
Your butt is a warm, moist place that is inviting to many kinds of bacteria that can cause bad smells. Also, some residue is left from your stool no matter how well you wipe. A thorough bathing will clear those smells right up.