COMIC URBAN LEGEND: A change in postal laws led to the elimination of letters pages in DC comic books.
In the early days of comic books, one of the more intriguing aspects of comics was the presence of text pages in the midst of comics. People used to have a number of theories as to why they were there, some figured that perhaps the publishers wanted to add a bit of culture to their readers.
The truth of the matter is, to qualify for the second class bulk mailing rate (the same rate given to newspapers and magazines), periodicals were forced to include at least two pages of text.
Therefore, an amusing side effect occured. Desperate for text strips, the publishers would often print whatever was given them, which has led to some DREADFUL fiction over the years (this was Stan Lee's first work in comics, churning out text pages for Timely when still in his teens).
However, as time went on, and fans became more involved in comics, publishers realized that letters pages could just as easily serve this purpose, so that is why comic books all went to include letters pages - so to achieve their mailing rate.
In 1996, the United States changed their postal laws, eliminating the "Second Class" mailing rate. Now there is just a single "Periodical" rate. A company can qualify for this rate by being just comic books.
DC Comics does not have letters pages anymore.
However, by 1996 (thanks to Steven Rowe), DC had already switched over to a different mailing rate, thereby negating any impact the change would have upon their decision.
In addition, DC did not eliminate their letters pages for another four years, and their stated reasons (The internet has become the easiest way to interact with fans and they figure they could save some money on having no letters pages) are quite reasonable (especially as DC fired two design workers at the same time they eliminated letters pages).