THE X-FILES changed history. While I was never a huge fan of the show (it was ok), it did mark the first successful hit Sci-fi series on American television that was a hit when it aired as opposed to being a "cult classic" that discovered its audience years later. Personally, I would like to give this mantel to the brilliant program QUANTUM LEAP, as it lasted several seasons and had brilliant writing. However, it was not an over the top hit when it aired, but NBC had much faith in it. (In an unprecedented move, the program aired five weeknights at 10pm in a row to garner an audience. The ploy worked)
 
Today, with the advent of the Sci-Fi Channel and other cabl outlets, there are a ton of Sci-Fi programs that are enjoying multi-season runs. It didn't always happen to be that way though. In fact, many of the classic and excellent science fiction programs of the network era were shortlived and, often, only modest successes with viewers when they originally aired. 

If you look at the television programs of the 1960's and 1970's that were successful, what becomes obvious is that their actual number of years on television were quite miniscule. THE TWIGHLIGHT ZONE lasted five years, but other hits were decidedly less successful: SIX MILLION DOLLAR MAN (4 years), VOYAGE TO THE BOTTOM OF THE SEA (4 years), LOST IN SPACE (3 Years) STAR TREK (3 years), SPACE: 1999 (2 years), BUCK ROGERS (2 years), etc. The volume of sci-fi shows that lastd one year or less (sometimes quit deservingly) in much more expansive. Think THE MAN FROM ATLANTIS, CLIFFHANGER, MAN CALLED SLOAN, and THE POWERS OF MATTHEW STAR. (On second thought, forget about THE POWERS OF MATTHEW STAR)


Matthew Star - 'nuff said

This, of course, begs the question as to why science-fiction continually failed on television and why networks would try and try again with new science-fiction programs. To answer the latter, and this is pure speculation on my part, I would say that because science fiction was so wildly successful in movi theaters television producers thought they could duplicate the success. Even if they could not attract a ton of viewers, they could make massive money from merchandise and tie-in products. (THE SIX MILLION DOLLAR MAN made millions upon millions in toys, games, etc while never being a huge ratings hit) Often, these experiments would fail and the programs would bomb in the ratings after being short lived. Honestly, when you stop and look at it, sci-fi was only used sporadically as TV entertainment when compared to sitcoms and cop shows. Perhaps sci-fi was used to try something different?

From this, we can see the main reasons that sci-fi bombed on TV back in the day: the networks did not have their heart into the genre and this often undermined the work of the producers, writers, et al. The networks really did not understand the genre and, more importantly, they did not respect the genre. Because of this, many programs were doomed from the start as they were misconceived, micromanaged and dumped into timeslots amidst poor promotional campaigns. Throw in a heap of bad writing and you have a recipe for total failure. This would be th case for science fiction on television until the advent of syndication into the UHF market would change all this....