Several reasons, actually. He brought to a struggling Fantastic Four a renewed sense of “big.” Big science fiction themes, big villains (Kang and Galactus, to name two). Big ideas. In this arc, he takes the team across the time stream to face all kinds of wild crazy stuff. But most importantly, he brings his genius sense of pacing, illustrated above. Being able to write dialog is great. Being able to draw pictures is great. But being able to combine the two so that it’s fun to read dialog–that’s art.
He also brought his great sound effects. See also: His work on Thor.
And how many creators can pull off a page and a half of black panels with nothing but word balloons?
This is a really stellar run. So many great ideas, so much fresh storytelling. Thank you, Walt.