Mainly because the HUD makes it pretty darn close to routine. We had the same setup in the Navy. Our C-40's (737-700's) also have a HUD. Although we are not certified to fly CAT III in the Navy you can easily hand fly a CAT III down to CAT III minimums even with a single engine - which we usually would do in recurrent if we had extra time. It is a very sweet system.
Our 737's at DAL are also CAT III capable via the HUD, and I am pretty sure these are certified for single engine operations.

Scoop