In my opinion it has to do more with the overall developtment of the warmaster lists than a simple translocation of WHFB to WM.
Making a "whatever" warmaster list is very simple, but making a balance and reliable list for WM is not easy at all. So this type of decision had to be made.
But if your question is: why drop archers? I do not know, maybe they have enough entries, or the use of goblins and wolf raiders with 15cm range weapons (as harash and skirmish army) could be annuled by proper archers.
Maybe bow is not a cultural weapon of the green people, so the abstraction of the army is going to close combat more than big units of disciplinated archers.
In warmaster you could not think as warriors, you have to think in military units. Can you imagine 300 orcs with bows waiting for a signal of the oficier to shoot. Can you imagine 300 orcs waiting until the enemy enter in the zone where the arrows could harm them. Remeber the use of bow as military unit is parabolic shoot not tense shoot, so the experience of the officier of the unit is important to measure the distance .
Does it means that orcs cannot use bows? I don¡t think so, I could imagine two orcs with bows and poisoned arrows ambushing a Empire pistoliers while passing a bridge. Empire pistolier is carrying an important message to be given to elector count,. But This situations is about other game...
