It all comes down to this: A book is a friend. A movie is a stranger with a problem.
You invest more of yourself (i.e. more money) in a book, you invite it into your house, you visit it multiple times for multiple hours, you have to actively commit to it in order to keep the experience going. But a movie is totally different: You invest less; you meet it socially, in a neutral location; you only agree to sit down with it once; and you can just sit there passively and judge the story, without committing to it. Therefore the movie (the stranger) must stick to the point, in a way that a book (your friend) doesn’t have to do.
Every scene in a movie has to be (at least tangentially) about one problem. After all, if you agree to sit down with a stranger to talk about their problem, then you will become very impatient if they suddenly start talking about something else from their life unrelated to that. There is an understanding that you’re only there to hear about one thing. Friends, on the other hand, can jump around and talk about a bunch of different problems or just make idle chit-chat about what they did all day. You like the sound of their voice, and if they drop a random tangent they can always finish it up at a later time.
The book “L.A. Confidential” had a massive sub-plot about the hero’s difficult relationship with his retired father. The screenwriter struggled mightily to include it, but it shrunk down more and more in each draft. Finally, it disappeared altogether, and the hero became an orphan. Wise move. There’s nothing worse than an adaptation that feels the need to include brief glimpses of every subplot from the book, even if they won’t have the time to play out in a satisfactory way.
As always, when writing a movie, think of that talkative passenger seated next to you on the plane. If he starts telling you a surprisingly involving story about some police corruption he helped to expose, would you want him to suddenly say, “Oh, by the way, at the time I was also dealing with a lot of issues with my father, so let me tell you about that, too...” Nope. You would say, “Actually, I’d much rather hear the end of the story you already started, if that’s alright with you.”