To attack from both direction enemy would have had to cross Misty Mountains, which was no more easy than to cross bottomless abyss by one bridge alone, or turn around the mountains from the Ssouth. In both cases it was not possible for such a great army as would be sufficient for attack on Moria to remain unheeded by any (either dwarves or elves leaving both sides of Hithaeglir). In later days Rohan was established to the South, and the task became even harder, even though Hollin was left desolate.
Besides, 40 miles is not such a hard task for stout dwarves, and in their own home-cave for an advantage, where they knew every corner, and enemy must have been blundered and stumbling over roots and things
