According to the seer — who is none other than the little girl named Robin from Season 4 who has become an Inhuman — Coulson, May, Daisy, Jemma, Mack, and Elena were needed on the Lighthouse in the year 2091, but Fitz wasn’t part of the prophecy.
Although he was sent to Earth to observe and record the evolution of humans, he began interfering with humanity when he came across a “seer” who could predict an extinction-level event.
The dynamic duo began investigating to figure out what happened to everybody in the diner, and their investigations led them to the home of Enoch, who just so happens to be a 30,000-year-old sentient being from the Cygnus constellation.