By the time he had walked to Mt. Doom, Frodo was exhausted, weak, malnourished, and had carried the burden of the ring for a long time. These all contributed to him succumbing to its power.
If the eagles had flown him, none of those factors would be at play. Frodo could have still had the strength to overcome the ring's power and throw it into the fires.
If the eagles had flown him, none of those factors would be at play. Frodo could have still had the strength to overcome the ring's power and throw it into the fires.