The "micro" reason is that the Berman formula that
ENT depended on for the first two seasons had long since lost its appeal to more than a niche audience of a few million die-hards who will watch anything as long as it has people wearing Starfleet uniforms.
The "macro" reason is that TV in general has been undergoing a long-term trend towards balkanization as audiences shift from networks to cable (Americans are actually watching more TV than ever). Metworks will never again enjoy the mass audiences they used to in the heyday of the 70s-80s, when you could have 80M people watching
Roots or the final episode of
M*A*S*H.
Nowadays the game is figuring out how to survive on a puny audience of 2-3M. But puny by whose standards? On premium cable, that's a monster hit. On basic cable, it's respectable. On network TV, it's cancelled. Yet that seems to be what people want - if you're not a truly lowest-common-demonemator show like
American Idol or
CSI, you better be able to survive on a niche audience because even if you premiere strongly, you'll steadily shed viewers till you hit that puny niche audience.
The moral of the story is that you can find an audience for pretty much anything.
ENT's audience was around 4M at the end, which is more than such well-respected, famous names as
Dexter, Breaking Bad, Damages, and
Mad Men. If
ENT was more popular than shows like those that run for years without any threat of being cancelled, then clearly it's not the content at fault. It's the business model. Sci fi is niche by definition so cable, not network TV, is where the business model makes the best fit.
TiVO and other DVR systems may or may not help in the ratings battle. Since 40% of ads are apparently not zapped, there should be some value to advertisers even for time-shifted viewing. It's hard to get a clear picture of this since it's spin city out there, with networks arguing the advertisers should pay full freight and advertisers digging in their heels and refusing to pay a cent. And everyone issues press releases and BS "studies" to back up their claims, so you'll read all sorts of stuff on the internet but you should take it with a bushel of salt - it's pushing someone's agenda but who knows what the connection to reality might be?
Paid and ad-supported downloads are still an insignificant fraction of the overall TV business.
I sometimes wish we were like England and had a tax for good programming rather than depend on advertising. Our PBS, our viewer-supported television station, makes very little and manages to bring mostly documentary programming. Ken Burns voice overs are a lot cheaper than high-quality actors and special effects.
You just argued against yourself. Why should my tax dollars go to PBS when they're not the folks who produce the
Futuramas, Fireflys, Farscapes, DS9s, and
BSGs I want to watch? Sure, the documentaries are nice, but are we really willing to rely on government bureaucrats to give us kick-ass space opera? Is that what they'd regard as "good for us" (and that would have to be the standard they use, to justify the use of our taxes.) I shudder to think of what kind of sci fi a government bureacrat would regard as "good for us" and sincerely doubt it would be worth wasting ten seconds on. I think something they'd regard as
bad for us would be a lot closer to the mark.
But what would really happen is this: the majority of people who aren't sci fi fans would howl bloody murder that their money is being used on some stupid crap about spaceships and aliens. They howl when they're money is used to get them
health care, can you imagine if anyone dared use tax dollars to make
Star Trek? The only stuff the bureaucrats would dare produce is the most bland, inoffensive, uncontroversial shows possible. All the stuff worth watching would continue to be on cable, which is really where
Star Trek needs to be if it's ever going to be worth watching.