The Great Eagle Mystery

[Gwaihir the Windlord]

While the storm on Caradhras may not have been anything to do with Sauron, it is true that

His arm has grown long indeed

and that he had to power to do such things. As I've said, the Nazgul were in all probability faster than the Eagles in flight, and certainly much better aerial warriors. Eagles were dreadful to those on the ground, but such birds of prey need to drop from a height and strike with talons. The Nazgul not only wielded swords and dragonish creatures that probably could fight in the air, but fear. A guard of Eagles would not have been much good, as the Nazgul would know immediately which one was carrying the Ringbearer.

It simply fits in with the point of the whole thing, that a full assault on Mordor is doomed to failure. Mordor was invincible to the powers that its enemies wielded.