TOS and TNG were the most popular TV shows, Voyager and Enterprise were the least popular TV shows. That simple fact might account for why we so many TOS/TNG novels compared to VGR/ENT. In fact, despite grumblings on this forum, the best selling Trek novels are TOS novels set during the five year mission, which is the main reason they seem to dominate novel line-up each year.
Also, there's aren't really that many DS9 novels published these days. Last year saw two, and they were under the Typhon Pact banner and featured the TNG cast in a supporting role. Whatever DS9 we get this year is going to be part of "The Fall."