In Transformers: Revenge of the Fallen, Jetfire reveals that only a Prime could defeat the Fallen.
So why is it that only a Prime could defeat the Fallen?
Well, The Fallen himself was once a Prime. Optimus is the only Prime left although he is not as strong as his ancestors, and that is why when he came back, he needed Jetfire's parts to defeat The Fallen. There are 2 versions to it: first is that, it's their nature law, that if you are a Prime no one can defeat you unless he(it)'s another Prime. The second one is just a prophecy that The Fallen can be beaten only by a Prime.