The following contains minor spoilers for Sicario: Day of the Soldado.
The original Sicario was a much-celebrated thriller and a large part of the film's success was due to the work of Emily Blunt. This made the fact that Blunt doesn't appear in Sicario: Day of the Soldado a problem for many. However, Soldado director Stefano Sollima explains that there's a very good reason why Blunt had to be written out of the sequel. Blunt's character of FBI agent Kate Macy was the moral center of the first film, and the sequel simply doesn't have a moral center. According to Sollima...
Emily Blunt is an amazing actress, but her role was sort of a moral guidance for the audience. In Soldado, we don't have that. This is closer to my vision of storytelling. I prefer not to have a moral guidance for the audience.
In Sicario, Emily Blunt's character was enlisted by the U.S. government in an attempt to take down the head of a major drug cartel in Mexico. However, it's clear from the beginning that she doesn't know everything about what is happening and the more she learns the more she realizes the characters played by Josh Brolin and Benicio del Toro aren't going to let things like morality, or law, stand in the way of completing the mission they've come to do.
Sicario: Day of the Soldado pretty much picks up where these ideas leave off. Josh Brolin's character is sent into Mexico by the U.S. government to start a war between drug cartels and the methods he uses to accomplish the goal are the sorts of things no government would ever admit to. The fact is that you can't have Emily Blunt's character return in a movie like Soldado, because if she was there she would either blow the lid off the entire operation or, based on how the first film ended, get killed trying.
Of course, the other problem is that, as screenwriter Taylor Sheridan had previously explained, Emily Blunt's character went through a complete arc in the first Sicario. There's really no reason to bring her back as there's just nothing the character needs to accomplish. She's also already been through hell once. What audience would want to watch her, and by extension, us, go through that again?
Of course, sometimes the story you have to tell is about amoral people doing amoral things and a moral guidance character simply has no place. Soldado is just that sort of movie. It's hard to argue that any character in the film is a "good" person, and Stefano Sollima tells Business Insider he doesn't think a moral center is necessary, but that doesn't mean the movie doesn't have a story to tell or a point to make. Although, Soldado isn't being embraced by the audience quite as well as the first film, so maybe a moral compass isn't a bad thing.