Good stage length - allows you to dedicate one aircraft to the route, there and back every day, without having it too tight thus unable to recover from delays, but without having the aricraft sit on the ground for too long between flights.

MIA was similar when it was the sole longhaul route BA operated out of T3 - a 747 would go there and back pretty much every day (well, for up to a week before being swapped out).

I believe New First was first introduced to ORD for this reason as well...