That must be one hell of a big DLC with the amount of invisible walls in F:NV :spotted owl:
If you remember Colonel, many people disliked "going off map" on DLC's especially BS.
As the map was full of detail already in FO3, it would have left some big issues in the time frame to implement a massive overhaul of mass battles taking place in the same areas. ( and would get boring. )
I think Obsidian have attempted to solve this by not only leaving a vast area to expand to, and areas marked by FT to be used as DLC Launch points.
But also leaving some pretty sparse areas in the main game map, that not only could modders make use of, but be incorperated into the DLC if a over reaching one was to get released.
So imo, and hopes I think they put alot of planning ahead in this, by listening to opinions.
Or they just left it to make the Mojave seem less like a box, and more like it had natural boundries, focusing on the places that mattered.
Either that or they over reached themselves, and had to cut a ton of product, and we will never see the greener nuclear glow across the mountain range.