Maybe because he's had a lot of his life being told not to get undressed and/or he struggles with the coordination and dexterity necessary to take his lower clothes off on a timely fashion. Maybe although he can strip just fine,he hasn't connected that he can get himself to naked to use the potty himself. He knows he can get naked. He knows he can use the potty when he is naked. But connecting those 2 things hasn't quite happened yet.