Though there was nothing riding on the visit, the first impression you project can affect the way staff and faculty interacts with you. Not saying, of course that a person in jeans and a T won't be taken seriously or will be ignored. Character matters more than anything. Wearing jeans wouldn't have NECESSARILY killed you, but I am quite sure that wearing a nice outfit on your first visit definitely helped reaffirm the decision the committee made. They know you care about the little details and that you are excited about SULaw/SeaLaw.