One possibility is that it may come down to money. In FCS, all scholarship offers are not created equal. He may have received a full scholarship offer from USD and a partial from NDSU.
There are a million other reasons why kids make decisions about schools, and only some of them are rationale. Maybe he prefers a smaller town. Maybe a coach told him that he could start for four years. Maybe he met a girl on a visit. Maybe he didn't get along with someone at SU, or the coach recruiting him hasn't paid enough attention to him. This is part of what makes recruiting high school kids "interesting."