Here's an interesting read from this very site (Trek Nation)
When TNG premiered in 1987, it was the only first-run syndicated show on television, and one of very few sci-fi shows in general. At the time, there were 4 networks and a handful of cable channels.
As TNG flourished in an atmosphere without competition, it firmly rooted itself in a position that would cause it to grow -- TNG was actually replacing some networks' prime-time lineup in places.
When TNG ended, this seven year downward spiral began, but not because Trek as a whole was getting worse or less-liked, but because each new show was starting in a crowded, competitive environment with many similar shows. As of June, there were 7 networks, dozens of first-run syndicated shows, and over a hundred cable and premium channels. Where TNG had to deal with maybe a dozen competitors, DS9 and Voyager contend with around 50 (counting the premiums) and a sci-fi market that's close to being oversaturated.
Where TNG was able to grow some roots before the major onslaught of competition began, DS9 and Voyager have grown up in an environment very different from the one TNG grew up in. DS9 had to fight for prime-time slots and Voyager was only seen by as many people as UPN could reach. With Trek's quick fade from the spotlight after the end of TNG, Trek swiftly lost its casual viewership and mainstream support. The number of people viewing Trek has shrunk back to what one would expect from a wildly successful cult TV show. Yes, that number is smaller than it once was, but for what Star Trek is, it's still doing quite well.
The Next Generation was basically a mainstream show that fully intended to be a mainstream show. Good mainstream shows get pretty high ratings as TNG did. DS9 was never a mainstream show and it never wanted to be one. Voyager has tried to be a mainstream success but simply can't (through no fault of its own) because UPN is such a failure as a network.
The Star Trek franchise may never replicate the mainstream success of TNG, but DS9 and Voyager have thrived even with the handicaps they have. Judging every Trek series against TNG is what is leading people to make grand claims that the Trek series is dying, but despite the fact that both are Trek shows, comparing TNG to DS9 or TNG to Voyager is like comparing Apples to Squash. TNG was born with a silver spoon in its mouth, DS9 and Voyager had to fight their way up.
Deep Space Nine spent most of its lifetime as the number one syndicated first-run show on television despite its falling number of viewers. Even when it became a near-serial show (usually, long-term serial shows are ratings disasters -- witness Babylon 5) airing in prime-time in less than 60 percent of the nation, DS9 managed well over a 4.0 average in its final two years. As a general rule, a syndicated show needs to maintain a 3.0 to be successful, DS9 always maintained that despite the strikes against it. Look at the other sci-fi shows similar to DS9: Earth: Final Conflict is regarded as a decent show ratings-wise, staying in the lower 3.0 range and Babylon 5 is the hot potato of science fiction television -- it's done so poorly that no one wants to hold on to it.
As a serial, more cultish television show, DS9 is right behind the X-Files on the all-time list of successes even with extreme disadvantages.
Voyager, on the other hand, has very little that it can brag about. That's not because Voyager is an awful, unpopular show, but because it's on an awful, unpopular network. Voyager can only do as well as UPN because of Voyager's status as a network show. UPN has been losing stations since day one and is now only airing in a little over 60 percent of the nation, meaning that Voyager is competing in a very crowded market with both hands tied behind its back. For its disadvantages, Voyager has still managed to remain UPN's top show. However, Voyager will never be able to perform near the level of its predecessors so long as it drags the carcass of UPN wherever it goes. That's not the sign of viewers losing interest in Trek, it's the sign that viewers aren't interested in UPN.
In short, the ratings are down. Less people are watching. The bleeding off of TNG's more casual Trek audience is nearly complete and the shows are losing some of their viewers to competition. What does that mean? Is Star Trek dying? No, Star Trek is not dying, it's just not a mainstream hit anymore. Things have grown far smaller, but smaller isn't necessarily bad as long as a show is maintaining minimum audiences, which so far Trek has done. Star Trek may someday stage a mainstream comeback with a TNG-sized hit, it may not. But as long as Star Trek maintains its loyal audience that it currently holds, the franchise will be fine.
This doesn't mention Enterprise but many fans don't like to talk much about it anyway. In the end Enterprise basically had no ratings.
http://www.treknation.com/articles/ratings_history.shtml
Which series is?
FX wise? I think they looked very cool.
