The Multiverse diverged the moment The Celestial Tiamut - Communicator of The Celestials - fell to the very Eternals that were tasked to bring him forth.
Ikaris, unable to live with what he had done, sought to end his life in the brilliance of the sun, but Arishem intervened at the last moment. 
The Multiverse was in trouble. 
Earth would soon be destroyed.
And the Universe would soon follow - every Universe would soon follow.
But there was a chance - a long-abandoned Celestial project. Where do you turn when the Multiverse is in trouble? Easy.... you turn to the Omniverse.
And who better to send than the most loyal Eternal?