Defense wins in the playoffs. That's why the Thunder breezed through the West and got slaughtered by the Heat last year. Miami struggled more with the Pacers and the banged up Celtics than they did with the Thunder. Philly, Atlanta, Chicago, Indiana, Orlando, Miami and Boston all have been bruising defensive teams that makes it so much more difficult to advance in the eastern conference.
If Duncan played in the east, the chances of him getting injured go WAY up. He won't be playing the Clippers, Warriors, Rockets and the like here. Offense wins games in the regular season, that's why the West has a much better record. In the playoffs the eastern conference is always tougher.