I think we all know what that "some reason" is. That is surely so by design. This is one of several (many?) routes where logical connections are "for some reason" never appearing (permanently blocked), and only much higher priced options are open. Very low percentage of pax will call and put it together segment by segment, esp. when many (most) agents don't know, or refuse to, do it right and price it as a through-fare, even though routing is valid.