Creating any adaptation of a book to the screen is a process of art that involves a number of players. The exact dynamics of the relationship determines which of these players has the most influence. Even so, the writer is the one who makes the most profound choices.
Those choices made by the writer have very little to do with respecting the material (even if they say they did) and a lot more to do with making something from a radically different medium fit into totally different shape.
Scripts follow fairly exact beats and an experienced screenwriter knows, to within at least a page, where those beats need to fall. As these constraints do not exist in the written form, sometimes a writer (or the director) may choose to cut out certain aspects in order to fit in everything else.
Then there is the matter of time. Films have to be of a finite and usually fairly fixed length. Take, for example, Lord of the Rings which is three large books. Prior to a well-known certain trilogy based on the books adaptations had been one or at most two movies long. That necessitates cutting a lot of material. Even with the extended edition a lot of material and whole characters and subplots, were left out.
The director may have a particular vision for the film he or she wants to make and may direct the writer to make changes. Ideally, the writer and the director should be on the same page but this does not always happen. Sometimes the writer is changed one or more times and although the first draft might have been based on the book, later drafts might not take the book into account. This can result in the script drifting in a whole new direction.
Additionally, the producer (and the studio) may decide they want to market a specific type of film and they may well demand that the adaptation be bent to fit the genre or specification that they are after. It is not unusual for a studio to demand a love story subplot because "they sell well".
Things get even more complicated if an expensive star actor is cast in a role that does not meet the studio's expectations for how much screen time that star gets. You can bet the studio, the star's agent, and the producer are going to be having words with the director and writer about giving the character more lines.
Finally, there are practicalities. As a writer, if I want a million orcs running across the battle field I write "A million orcs thundered towards them". For the director, this is not so easy. He needs to find a very large number of extras, use a lot of expensive CGI or find some way to have that battle happen off screen.
TL;DR: A novel is a square peg, the screen is a round hole. To produce an adaptation of a book for the screen, a lot of clever people have to figure out how to "make it fit".