Quite simply - there is much more land mass in the Northern Hemisphere, and flights, say from Moscow to Anchorage or Reykjavik to Toronto have great circle routes that traverse the Arctic circle.

The Southern Hemisphere on the other hand, has no large land masses near it, and even the southern most points of Australia/New Zealand and South America are too far away for any great circle routes to cut through there without being a greater distance than any other routing.