Sometimes, viewers are so afraid that a show will be Screwed by the Network that they refuse to watch it, even if it sounds appealing to them.
The Firefly Effect refers to viewers being afraid of committing to a new series because they don't believe the series will last long enough to make up for the investment of time and emotions. "The network is just going to cancel this, so I'm not giving it my heart." If enough viewers think this way towards a particular TV series, it may become a Self-Fulfilling Prophecy people don't watch because they think the show will be canceled, and then the show is cancelled because no one is watching it.
Television executives don't tend to want to invest in intelligent or overly complex series, because they worry that the given show will be less popular, (and thus draw in less money from ratings) if viewers are required to use their brains. As well as the trope namer Firefly, Star Trek: Voyager was a quintessential example of this trope it had an initial premise which was very similar to that of Firefly in some respects, and although the show didn't get cancelled, said initial premise was basically scrapped... along with the series being made a lot less intelligent and a lot more violent.
It can be a real problem for all the Lost clones and other Ontological Mysteries, which aren't worth watching if they don't get more than one season, and especially if they don't even get a full season and they often don't. That is also the sort of TV shows most likely to be hit by this effect, but it can hit any show that makes it clear up-front that you need to be involved in the characters and/or overarching plot to make sense of the show in the long run. That up-front demand plus the uncertainty that there will be a long run allows The Chris Carter Effect to start before the TV show does, meaning the fans never start watching...
In contrast, people usually don't think that they'll get overly attached to Crime Procedurals, Sitcoms, Soap Operas, or even Reality Shows; thus, they'll feel free to watch episodes "casually" until the attachment to the show (or characters) sneaks up on them.
Even some shows that seemed feasible only over one (22-episode) season (such as Reunion, Day Break (2006), Kidnapped, Vanished, and Drive) suffer from this effect, due to the episode order getting shortened to 13. In Reunion's case, the producers didn't even bother finishing the storyline, because it only made sense over a full 22 episodes rather than the shortened 13
Incidentally, many of these shows (including Trope Namer Firefly) were on Fox basically because Fox was likely to give the sort of show that gets this effect an initial run, but tended to be too Nielsen-sensitive to be patient. Fox has recently been experimenting to test the nature of this effect, as evidenced by Fringe (which seems to have escaped this trope, airing for five seasons) and Dollhouse (if this trope can apply to a show that has run for more than one season, then Dollhouse is it).
This is not yet a deprecated trope. But the rise of streaming and premium cable means cancellation - while not unheard-of - is less likely to occur before a major story arc is completed (i.e. a series with a six-episode story arc being cancelled after two episodes). For example, Jessica Jones (2015), released through Netflix, was never in any danger of being cancelled halfway through its first season, allowing fans to invest in the show without worry. Similarly, the streaming model allows viewers to choose how they invest their time. Someone who might not have watched one half-hour episode per week for an entire month to decide whether a new series is "worth it" will happily burn those two hours in one night. And it gives fans the flexibility to wait for word of mouth about a particular show before investing their time, though only to a point. Streaming shows have been cancelled due to lack of views within a certain length of time.