Books and movies are fundamentally different media, and require different storytelling techniques
Movies are better at conveying some things, like action or background events, but much worse at conveying other details, such as private character thoughts and expository details.
Also, movies have much less space for content than books do. Your average 100k word book does not fit in a 2 hour movie without significant trimming. Some things have to be cut.
The bread communication was intrinsically difficult to portray
The whole point of the bread was that the characters were not free to speak openly. That means that we would need some kind of internal monologue in order to explain the significance of the bread. An internal monologue or narration isn't something you can just add for one detail - you'd need to use it throughout the story, and it would significantly change the feel of the movie.
Something had to be cut for space reasons, so they chose this detail that was ill-suited for the screen as one of the cut items.