Going to try to disentangle a few aspects of what I was saying earlier, which partly due to my own lack of elaboration may have been conflated.
I understand we're probably mostly all
Trek fans here, given where here is, but regardless of whatever any of
us thinks about the quality of the spin-off shows beyond TNG, I'm not sure a good argument can be made that the mainstream non-fan audience would consider DS9-VGR-ENT and the later films "the height" of the franchise. By that phrase I didn't mean the period when there was the most
Trek being produced by volume, I meant in terms of its mass appeal to the widest audience, as shown by
averaged ratings. I think a better argument can be made that the franchise was on the whole in
decline once you get to the last couple seasons of TNG and beyond, and while the overall-increasing level of interconnectedness and self-reference surely doesn't remotely represent the
totality of what went into that, it seems logical and likely to me that it was a significant factor.
Regarding TOS/movie characters guesting on TNG, remember it was
that show which was initially the spin-off! That Bones cameo in "Farpoint" was the equivalent of Fury showing up on
AoS to give it a boost, and stories like "The Naked Now" and "Sarek" were the equivalents of Coulson's team finding a HYDRA artifact from
The First Avenger or crossing paths with Lady Sif! The other examples cited by
@bbjeg and
@Turtletrekker above date to the period when, as I said, the general viewing public's interest in the franchise was in steady decline, and even if they weren't solely or directly responsible for that trend in the first place, then at the very least they certainly didn't help to arrest or abate it! (And some might say the "why is Worf here
this time?" ones are good examples of just how clumsy and silly and handwavey such token references can come off.)
Conversely to pre-Abrams
Trek, the Marvel CINEMATIC Universe is a major motion picture franchise that has spun-off into some TV series, which are allowed to play around in the films' sandbox insofar as they abstain from stepping on the toes of the movies, which are the real "cash cow" keeping it all going. They lead, and the TV shows follow, and that dynamic isn't going to fundamentally change for a variety of reasons including the logistic ones explained earlier by
@Christopher, and also because that's just part and parcel of way the whole arrangement is
designed to work. And that's as it should be, because thus far it's working fine.
Now, would the popularity of the Marvel films suddenly and precipitously collapse and go all to hell immediately if they started dropping occasional references to AoS here and there? No, probably not, and that isn't really what I'm arguing. My point is more that from the perspective of those who make the films there is no advantage to things moving in that direction, and thus no motivation to take such a risk. They have quite enough balls in the air to concern themselves with already. And the only audience to whom adding more TV references would plausibly make the movies more appealing are the ones who are
already watching the TV shows! (And who, by the way, are
also already watching the movies.)
And to return to Guy's point:
Imagine if half the audience that saw the movie, decided to watch the TV show?
Imagine that if everyone who bought the CIVIL War DVD, then bought 3 seasons of Agent's of S.H.I.E.L.D. DVDs, because the movie hinted that cool and important things were happening on the small screen.
I imagine that if "cool and important things" were hinted at in a movie I paid $10 or more to see, but then weren't followed up on at all in it, instead requiring me to buy 3 seasons' worth of DVDs, I would find it offputting and annoying.