DL still has a few domestic-hub-international routings, though mostly on 738s or 757s, albeit way less than it used to be. At least for routings international-hub-domestic, the reason I often heard were the tight immigration rules after 9/11, which made it useless to have such direct flights, due to factors such as higher risk of passengers not catching their "direct" onward flight due to intensed screening etc. and of course the immigration rules themselves.