Because continuity isn't the reason we tell stories. We tell stories to explore interesting ideas. It can be interesting to explore an evolving, interconnected continuity, but the continuity is a means to that end, not an end in itself. If someone has a great idea for a story that can't fit with that continuity, they shouldn't be forbidden to tell it.
There is always a way to make the idea, you might just need a diferent sctor of space or a different era, thats all.
There are so many things wrong with that assumption that it would take all morning to go into it. But did you miss the part where I said continuity is not an end in itself? It is not something that writers should be required to obey. That's an arbitrary limitation that has nothing to do with quality. Saying that all stories need to be kept slavishly consistent with each other is like saying that all stories need to be musicals or that all stories need explicit sex scenes. Continuity is a stylistic option. It's a technique that works well in some contexts but absolutely should not be the only permitted approach.
The writers and editors developed the interconnected Trek novel continuity because it was fun and interesting, and because we wanted to pay tribute to other authors' works we respected and play with their ideas and characters further. But if the best way to tell a story is to move in a separate direction from that continuity, then it would be wrong to force that story into a mold it doesn't belong in.