No offense meant to Dayton Ward, as his Trek novels are some of my favorites (especially the VAN novels), but I can't understand why this is a story in its own right rather than a subplot / scenes in a TOS novel.
I think Dayton already kind of answered this -- he was approached by Pocket Books to write this particular e-book-only novella. That was the request from the publisher; it wasn't as if DW wrote the novella and then submitted it.
It appears that Pocket is testing the waters of e-book-only content, so they're starting off with new stories spinning off recent, well-received series, such as Typhon Pact
. The series connections are marketing, not authorial whim.