I suspect that it has to do with timing.
Several years back (early nineties?), Northwest committed to refurbishing/retrofitting their existing fleet in lieu of purchasing new aircraft. If you'll remember, Northwest had some pretty severe balance sheet problems during the Cecchi regime.
Now, they are in a situation where they know they will have to retire their DC-10s within the next few years. Today the 763 must compete with the A330 and 777. Given NW's success with DC-10 on European routes, and given the capacity of the DC-10, the A330 or 777 might make more sense.