I am not following you here. If, as you say, "culture, taken in a broad sense, explains almost everything about which societies become economically successful and which don't", then the causality there would be from culture to IR, IR would therefore be dependent on culture, and IR should pay attention to culture. Are you saying that cultures are influenced by ideas from other nations? That seems plausible to me, but that would not make it an either/or situation (either IR does not get culture or culture theory does not get IR.)