[Formendacil]

It's a good question.

The first thought that I have is that the Witch-king--for whatever reason, I can't think of one right now--couldn't send the Wights into the territory still held by the Dúnedain--meaning that the Shire was out, Fornost was out, and Bree was out.

Perhaps, in that case, the Barrow-downs were simply the best spot to put them--a way of hedging in Arthedain's southeastern frontier, and reinforcing the dread that the Dúnedain probably already felt for the scene of Cardolan's final defeat. Despair, after all, was one of the more potent weapons Angmar could wield, and as Arthedain became more isolated and their resolve broke down, that could be quite effective.

It also occurs to me that the Wights may not have been worth much as an offensive force. They seem unlikely to stand up in full sunlight in the midst of battle. Perhaps their presence was more effective on the edges than at the centre of things.

Of course, the Old Forest was already there, so one wonders about the duplication...