The problem facing editors abridging books for reading, like dramatists adapting books for stage or screen is time. To read the whole book would take weeks, if not months, so they have, so to speak, to cut to the chase and just follow the plot.
I think this is why adapting Jane Austen's for stage, screen or radio is so popular. Her books are plot driven with few sub plots and it is easy to reduce the story to its key elements and dramatise that. Trollope and Dickens are popular for the same reasons, although they do have a lot of subplots, but these can usually be excised without damaging the main plot.
The difficulty comes with books, like Anne Tyler's and many other modern writers where the development of the character is key to the book, or where the characters have a rich interior life. To cut to the plot is often means removing what makes the book special. An interesting author is Mrs Gaskell, where 'Wives and Daughters', a predominantly plot driven book has been successfully adapted for different media several times, while 'North and South', a more character interior and emotion based booked has never, in my mind, been successfully dramatized.