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 action-packed.
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 (2007)) 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 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).
With the rise of Netflix and other streaming services during the The New '10s, it was initially believed that this would no longer be a relevant trope. However, during the late 2010s and especially during The New '20s, streaming - and Netflix in particular - would develop a reputation for cancelling shows after one or two seasons because the viewership wasn't as high as they wanted, often leaving those shows on unresolved cliffhangers. It's gotten to the point where Netflix has become even more infamous for this sort of thing than Fox ever was.
This also applies in videogames, especially live-service and freemium ones. There are companies ranging from Nexon, Square Enix, and Crunchyroll that were infamous on closing numerous free-to-play games in span of less than five years (and some even only having chance to go live in a year or two) that cautious players began avoiding those by companies infamous of doing these.