Why do writers keep ruining potentially good adaptations?

I dont get why the effort of including side stories that were not in any of the movies, but yet continue to change them.

From tiny details up to entire stories

For example, Caderrouse part of the revenge plot or the Telegraph? Why changing small details that adds nothing to the story? Why knocking the operator out instead of bribing him? Whats the point? Adds nothing, yet, they had the opportunity to keep it faithful to the book. Simply choose not to.

Not to speak about the endings….. 6/10 because of stubbornness of the writers.