I think that part of the answer lies in the notion of Sophocles taking dramatic license. Sophocles is more concerned with the exploration of themes and characterizations of figures like Creon and Antigone. Sophocles understands that showing the burial of Polyneices might detract from that. In showing the burial, it takes away from the characterization of Antigone. Sophocles recognizes that the characterization of his heroine is one that must transcend all else. If he shows the actual burial, it takes away from the thematic importance of what he wishes for Antigone. At the same time, I think that Sophocles recognizes that the actual burial is not necessarily important. It is the mere mention of it that sends Creon into a rage. To a large extent, Antigone's protest is not over the burial as much as the denial of discussion as to why Creon's adjudication is above reproach or questioning. In not showing the burial, it maintains Sophocles' dramatic license to argue that the notion of voice and dissent is something intrinsically valuable and important. In doing so, not showing the burial actually gains in thematic importance and relevance.