It's no secret that Marvel's first TV series, Agents of S.H.I.E.L.D., got off to a rocky start when it debuted in 2013. Despite initially breaking records, the series' ratings dropped off, and they never quite recovered. The series has since found its footing and a steady stream of viewers—and more importantly, it's become a tool for Marvel TV to introduce new elements into the MCU, namely the Inhumans.
As comics fans are aware, the Inhumans are the result and remnants of experiments conducted by the alien Kree race on ancient humans, with the hopes of building a superior race. Millions of people in the MCU contain the Inhuman gene, and by being exposed to Terrigen Mist (the same substance used by the Kree in their experiments), they undergo Terrigenesis, cocooning in a rock-like exterior before emerging. In many cases, they appear different. If not, then they at least gain some sort of Inhuman ability.
By bringing the Kree into the fold, Agents of S.H.I.E.L.D. was able to explain how Agent Coulson is alive (despite dying in Joss Whedon's The Avengers), as well as introduce the Inhumans. Ever since the superhuman race debuted on the show in 2014, they've become an integral part of the overarching narrative. It would be strange for Marvel to suddenly ignore all the development Agents of S.H.I.E.L.D. has done on the Inhumans in favor of reintroducing them on the big screen, to a wider audience. It wouldn't make sense, narratively.