If they wait too long to release the books, couldn't that hurt the books somewhat? After all, they were written with only Trek XI to draw from. If they're released after XII or later they would clearly be a product of another time and things have developed much from when they were written. It could be similar from a novel written during TNG's first season being released during the sixth. Thing are very different now and it would show.
Granted that doesn't mean the book wouldn't still be a worthy read, and some might even see novelty to it written with only the first season to draw on. But that book could have been more popular in the first season than it would be now in the sixth.
Maybe I'm misunderstanding the situation, but everything about how these novels were handled just seems sloppy. Especially now that we have a comic series telling new stories and a video game coming.