The motive for these deletions is abbreviation. But the form which the deletions take is governed by the physical process of copy editing. Editors favour changes which are easy to make on a page. Re-ordering material is disfavoured; minor insertions are more acceptable; and words or phrases that can be crossed out and still leave a good sentence are a gift to the overworked copy editor. There is probably no time to retype the copy before the next deadline. If something can just drop out and the linear surface be rejoined, it is much faster than shifting bits of sentences around.