The triangle's broad base at the top of the figure represents the most substantial, interesting and important information the writer means to convey. The triangle's orientation is meant to illustrate that this kind of material should head the article. The tapered lower portion illustrates that other material should follow in order of diminishing importance.
This format will allow the less important information to be more easily cut out of the article to fit a fixed size (number of words, printed size, etc.,).
The hourglass is another figurative paradigm for text construction.