Because Pocket Books has a history of doing Star Trek series that are thematically linked rather than ones that present a single narrative.
Star Trek: Section 31 did not tell a single, unified narrative. Neither did Star Trek: The Lost Era, or The Captain's Table, or Invasion!, or Worlds of Star Trek: Deep Space Nine, Day of Honor.
Obviously, they've done plenty of novel series that did tell a single story -- Destiny, Gateways, Terok Nor, etc. But it's never been automatically the case.
Like Christopher said, there is no Typhon Pact arc -- anymore than there was, say, a Cold War arc to James Bond or John le Carré novels. The Typhon Pact is the new status quo, not the new problem to be solved.
By all indications, the TP novels tell a unified (though understandably large in scope) narrative - the TP novels are happening in a (more or less) chronological order, with plotlines spanning several novels. Further, there are no non-TP-related novels from the participating series
I really don't think this assertion bears much scrutiny.
Now, obviously Plagues of Night
is touching on or advancing plot points from previous Typhon Pact
novels. But the other TYP stories -- I'm using "TYP" as my abbreviation, both to fit with the standard three-letter abbreviation and because "TP" always makes me think of toilet paper.... -- were not interconnecting in any meaningful sense. Zero Sum Game
was a self-contained DS9 novel that left the door open for continued Bashir/Serena stories, but which had seemingly resolved the quantum slipstream arc. Seize the Fire
was a self-contained story that hasn't been followed up on at all. Rough Beasts of Empire
was a DS9/TOS novel with its own self-contained narrative about the fall and rise of the Romulan government. Paths of Disharmony
was a TNG novel that advanced an arc from the DS9 Relaunch and hinted at events from Star Trek: Vanguard
. The Struggle Within
was an episodic TNG novella that hasn't been followed up on at all.
Meanwhile, one of the key political developments from the TYP series, Andor's secession, is apparently going to be more followed up on in the next TTN novel, Fallen Gods
, than it will in an actual TYP novel.
Even if that's not the intent, the novels from 2009 onward dealing with the TP tell an arc of sorts,
Not really. The only "broad" TYP arc you could construct would be "The Typhon Pact tries to gain a quantum slipstream drive," and that arc only applies to Zero Sum Game
, Plagues of Night
, and Raise the Dawn
. There hasn't been a single story (with an introduction, a rising action, a climax, and a resolution) to TYP apart from that, and that doesn't apply to four out of seven novels.