There's also the issue of what is unique about that era that justifies setting a story there? Decker and Ilia are gone, Saavik isn't on the scene yet, and everybody else (except, okay, Chapel) is back in their usual chairs.
Except for Chekov, who's become the security chief. L.A. Graf's post-TMP novels focus heavily on Chekov's security work.
Also, Spock is profoundly changed, because he's no longer denying his emotional side but learning to integrate emotion and logic, moving toward the serene, self-assured person we see in TWOK and afterward. To me, that was the most compelling thing that made the post-TMP period worth exploring, and it always bewildered me that nobody bothered to build on it until I wrote Ex Machina. I mean, it's the single most transformative event in Spock's entire adult life -- including his death, since he bounced back from that without any major personality changes that I could see.
Not to mention that you've got that delightfully rich multispecies crew that TMP just gave us the barest glimpse of. That's so much more interesting than a crew that's all humans except for Spock, Arex, and M'Ress.
In theory, any post-TMP story ought to be set there for some plot-specific reason as well . . . and not just because there's a gap in the timeline.
Now, that I agree with. Another reason I was so eager to do Ex Machina is that so few of the post-TMP books (other than the Graf ones) really felt like they took advantage of the setting. Most of them could've worked as 5-year mission novels with only a few tweaks. The Marshak/Culbreath novels tried to take advantage of the post-TMP setting; The Prometheus Design featured elements like the clothing replicators in the sonic showers as plot points, and Triangle built on the reference to "New Humans" in the TMP novelization (although it also depicted Zaranites as looking like humans for some reason). But TPD dealt with Spock's emotional epiphany by arbitrarily undoing it, having Spock actually become more emotionlessly Vulcan for no clearly explained reason. So I never felt they really took advantage of the potential of the setting either (leaving aside their other problems).