[Маг]

В книге сказано следующее:

Here, - said Balin, - in the old days we used always to keep watchmen, and that door behind leads into a rock-hewn chamber that was made here as a guardroom. There were several places like it round the Mountain. But there seemed small need for watching in the days of our prosperity, and the guards were made over comfortable, perhaps - otherwise we might have had longer warnings of the coming of the dragon, and things might have been different.

Фильмовая история, полагаю, такая же. Не было нужды кого-то высматривать, вот они и оказались неготовы к угрозе.