FAQ / Why did not the Eagles carry Frodo to Mount Doom
The most obvious reason here is that there would be no Lord of The Rings story and it would be over pretty quickly even if the Ring never made it to MountDoom.
On a more serious level, the most probable reason is that the Eagles are the
sacred birds of ManwŰ, and ManwŰ only very
subtly helped out, as he had to let the people of Middle-earth, namely Men to be able to fend for themselves without the
immediate help of the Valar.
Authored by TTFMember:Celebth˘l
Deus ex machina the 'god from the machinery', has its origin in the Greek drama where it meant the timely appearance of a god to unravel and resolve the plot. The name deus ex machina was probably chosen, because the god's appearing in the sky (or from above), was an effect which was achieved by means of some sort of a crane (the 'machine').
Tolkien indeed seems to have used the eagles as dii ex machina in his tales, in a Letter he wrote:
|The Eagles are a dangerous 'machine'. I have used them sparingly, and that is the absolute limit of their credibility or usefulness.|
This, IMO, also explains why Tolkien felt that he could not use the Eagles for such a crucial task as destroying the ring...