Back in the days when a large part of Usenet was implemented using dialup modems and uucp, the routing was set up to optimize the cost of the calls: machines called machines that were nearby to take advantage of free or flat-rate calls, and when machines needed to make long distance calls they'd do so after 11pm when the rates went down. Thus, it could take days for articles to propagate to the edges of the network.

Also: Interesting discussion of this article from a couple of years ago: https://news.ycombinator.com/item?id=11321236