Part of the answer is that the premise isn't quite right - "all the other Treks" didn't capture a huge audience.
should be looked at in the context of all television, not as if it's a special phenomenon that works by its own rules in terms of audience acceptance.
TOS probably had a big audience on NBC, when measured against the kinds of ratings the later syndicated Trek shows achieved - with only three networks dominating the prime time schedule in the 1960s, even the least successful shows had a lot of eyes on the screen.
TNG was the only post-TOS Trek that commanded a large audience for very long, and it's always hard to say (okay, impossible to say for certain) what the variables are that make one show popular and another less so. It almost always seems to have something to do with the audience really liking the actors, and there's no formula for this (just because - for instance - you may like Auberjenois better than Spiner or I find it painful to watch Charlie Sheen and Jon Cryer together means nothing
; other people seem to lfeel differently).
From the POV of the general audience the post-TNG shows are purely and simply TNG sequels, each one a little more removed. Sequels to immensely popular TV series do not have a history of necessarily great success: M*A*S*H
, Mary Tyler Moore
. The tendency, looking backward, may be to see Trek in the context of later successful franchises like CSI
or Law & Order,
but that kind of "branding" was an innovation which had not, up until then, been made to work. And - based on examples like the above - Trek managed to succeed to a greater degree with DS9, Voyager
and even Enterprise
than most other sequel series up to that time. About five hundred hours of post-TNG Trek was successfully produced, after all.
One can also cite examples of sequel series that were
successful, but this just brings us back to Goldman's dictum: nobody knows anything
. Most new series fail or fall short of hoped-for success, and it's rarely because the producers and financial backers didn't absolutely believe they had it right based on their industry experience and careful observation of the market.
Really, a better question would be "why was TNG so successful to begin with?" and that, despite our enthusiasm for the Franchise, has no better answer in hindsight than "because people liked it a lot."
Each Star Trek series that succeeded TNG fell a little further down the curve of diminishing returns. DS9 started out with higher ratings than TNG for a few weeks, then started to fall off. Voyager
spiked for a couple of weeks, then fell onto the same curve, likewise Enterprise
. When the curve of audience attrition bottomed out somewhere around two million viewers a week, it was over.
(Above is partial data, but it demonstrates the curve. I'm missing a chunk of one Voyager season, there, and I dropped the Voyager
and DS9 premiere weeks just because at the time I made the graph my mistaken reasoning was they made the falloff for both shows look worse than they actually trended - DS9 in particular spiked very high for its first week and then fell a lot in week two. Data for TNG, DS9 and Voyager
is sweeps data; Enterprise
is week-by-week. I believe someone on the board has a complete week-by-week graph that makes the point more definitively; I can't find it in the archives but hopefully someone will post it).