The reason is simple - historical accuracy . When we publish an issue of TidBITS, we consider that issue set in stone afterwards, and the only change we consider acceptable in our database and archive of issues is one to fix a typo or misspelling. At least with a publication, it's simply too tempting to go back and change articles to reflect what may have happened after deadline. If someone reads an article, then later finds it in a search after it has been modified, they stand to be confused. Think about an email address.
People move, addresses change.
That task alone would require a huge amount of constant vigilance, to say nothing of the many URLs that have appeared in issues since the Web came about. Plus, from a historical standpoint, it's dangerous to go back and change content. Historians rely on originals and successive drafts at times, and although the computer has eliminated most drafts, we can at least save our originals so people have solid information to look back upon in the future. A large software company who shall remain nameless once released a press release announcing that some product would appear in April. April came and went, and in May, we checked that press release to verify that the product was late, and lo and behold, the release date had changed in the press release on the company's site.