Warning: Huge Spoilers Ahead for _Star Wars: The Last Jedi*_. You don't want this movie spoiled, so don't read ahead until you've seen it. *
One of the biggest complaints coming out of Star Wars: The Force Awakens was how the film handled Gwendoline Christie's Captain Phasma. The villainess didn't have a ton of screentime in Episode VII, and she was unceremoniously put in a trash compactor on Starkiller Base towards the end of the film. When she managed to escape that ordeal, fans hoped to see her carry out some serious vengeance and have more of a story in The Last Jedi, but it was not to be. Captain Phasma perhaps has even less screentime in Rian Johnson's film, and is again dispatched, this time for good(?). It turns out there just simply wasn't a place in the film for a Captain Phasma story, as Rian Johnson explained.
The notion of having a side plot of Phasma throughout the course of the film --- look, I love Gwen [Christie]. I love Phasma. It would have been fun. But it just wasn't the story we were telling. There just wasn't a place for it. We already had quite a full plate to deal with in terms of all the other characters.
I think Rian Johnson touches on an important point here. It would have been fun to have a Phasma side plot, but fun isn't the same thing as necessary. The film is already a bit bloated, and forcing in a Captain Phasma subplot simply because fans like the look of her would only add more fluff that served no purpose. The Last Jedi already had to service the characters that were important to the overall narrative of the film and the saga. Detracting from them to tell a Phasma side story simply because everyone loves Gwendoline Christie's ass-kicking Brienne of Tarth on Game of Thrones is not good storytelling. The temptation to force something because it would be cool might be how other blockbusters operate, but it's not how Star Wars should. When asked by Business Insider if perhaps there was some Phasma goodness left on the cutting room floor to be enjoyed in deleted scenes, Rian Johnson shut down that hope.
There wasn't a ton of Phasma that we shot for this. The God's honest truth is, if you take a look at the movie, it's so full already. There are so many characters to serve already, and it's tough because Phasma really enters the movie when she needs to, and she does exactly what she needs to do in it. She's someone at the tail end of Finn's journey that represents his past for him to have this cathartic moment of being on the side of good and fighting her.
Star Wars: The Last Jedi showed that Rian Johnson isn't interested in mythology building. Every choice he makes is done to serve character and theme. Phasma's existence is meant to serve as a foil and a symbolic obstacle for Finn to overcome, and his victory over her is the manifestation of his growth as a character. For Finn, defeating Phasma means letting go of his past and finally, definitively choosing a side. If there is one thing Rian Johnson made clear with The Last Jedi, it is that he is not creating a film by committee with the vast Star Wars fanbase. He told the story he wanted to tell, regardless of the desires and expectations of others. And not all of those expectations are equal.
It is understandable that fans may have an issue with Rey being a nobody or Snoke getting cut down before he ever really mattered. Fans feel they were promised some things by the previous film that wasn't delivered on. They don't believe the setup paid off in a satisfactory way. But we were never promised anything about Captain Phasma, and no in-story elements led us to believe she would play some major role in the future. Captain Phasma was a character who grew out of discarded Kylo Ren design, and she immediately stood out to fans in the marketing for The Force Awakens. A badass woman in chrome stormtrooper armor, who never removes her helmet, had all the makings of an iconic Star Wars villain. Comparisons were drawn to Boba Fett and fans wanted more of her. But these desires were not borne out of any belief she was integral to the story of this saga. The majority of the enthusiasm around Captain Phasma came from her look and the love for the actress who plays her.