I don't know about this particular paper, but there's a) usually a page limit when publishing and b) definitely a limit to a reader's patience. So filling a paper with trivial or technical proofs is not good form.
That being said, many scientific papers would benefit from being rewritten in a style that focuses on the reader and readability. That's only understandable: when writing for a journal or conference, there's a lot of time pressure and your audience are bespoke experts on the matter at hand. But there's only a slight chance your paper becomes important to a broader audience. I wonder if one could form a business of rewriting classic papers of CS and math.