Sophie admitted she won the role of Jean over another actress (believed to be Saoirse Ronan) due to her social media following:
“I auditioned for a project and it was between me and another girl who is a far better actress than I am, far better, but I had the followers, so I got the job,” Turner told Porter, per The Telegraph. “It's not right, but it is part of the movie industry now.”
https://www.vanityfair.com/hollywood...r-social-media
Film studios want metrics they can use to decide on casting. Studio execs think the lead actors have much greater role in deciding whether people go see a movie and they can use social media as a metric.
But Sophie's social media following isn't just because people want to see her in movies. Most of the Game of Thrones cast isn't active on social media, but Sophie, Isaac and Maisie have been since the beginning of the show.
The casting for Game of Thrones was done by Nina Gold who's gone on now to cast the Star Wars sequels and is going to be credited with discovering a large number of well-known actors. So, casting Sophie over Saoirse or a more established actress wasn't out of the ordinary. Plenty of young actors are discovered on tv shows and go on to large roles immediately. The problem is Sophie is playing Sansa (a pretty annoying characters who for most of the series viewers rather hate
due to scenes like this in Season 1 of GoT) and Sophie was cast as Sansa due to her chemistry in a screentest with Maisie Williams (her BFFs who's playing Rahne in the New Mutants movie).
I think film execs can't figure out why MCU movies succeed and people love the actors in the roles, but other franchises aren't as successful. They want to use social media followers as a way to determine if people "like" an actor. But I'm not sure people actually really like Sophie or like the fact she posts pictures from the Game of Thrones set on Instagram.
IMO, Sophie was miscast as Jean, but I'm not sure anyone else would do a better job due to the way Jean is being written.