Why do you think they got so far behind didn't relaunch
Start with DS9?
The DS9 books followed the precedent of the TV series in telling an interconnected, rather serialized narrative. It proceeded at its own pace as determined by the needs of the specific stories it was telling. Other series that began in the post-Dominion War setting, including
SCE/Corps of Engineers and
IKS Gorkon/Klingon Empire, also proceeded at their own pace from their beginnings in early 2376, and so they were spread out across '76 and '77. Meanwhile, the
Voyager post-finale novels started after "Endgame" and thus were set in 2378. By the same token, after
Nemesis came out and moved the "present day" of screen Trek forward to late 2379, the books that spun out of it --
Titan, the TNG relaunch, and
Articles of the Federation -- naturally took place in 2380. In short, all the series' different timeframes were a function of what parts of the TV/movie franchise they branched off from.
So when it was decided to do a big, galaxy-shaking crossover trilogy (
Destiny), the natural place to put it was after 2380, so it could have real consequences. Most everything was jumped forward to its 2381 timeframe (or at least characters from dormant series like CoE and G/KE were shown in the trilogy), but DS9's editor Marco Palmieri wanted to keep its involvement to a minimum since he still had a complicated ongoing storyline he was developing and preferred not to jump it forward. Although at the same time, he
was willing to let
Voyager get jumped forward to the post-
Destiny timeframe, because it was in the process of being revamped and restarted under a new author anyway. It was simply a question of what, in the editors' judgment, worked best for each individual series.