Generally, a network will cancel a TV show that does poorly in the ratings. If there are no viewers, then there’s no way to get enough ad revenue to continue airing it. The occasional prestige, critical darling sort of show aside, networks rarely cancel anything that is doing well.
That would be why it was a surprise to fans when the Sci-Fi Channel canceled Farscape after four seasons. At the time, the show was the network’s biggest hit. It was also the most expensive, and it showed on the screen.
One of my favorite sci-fi space operas is Farscape. A co-production of Australian television, the Sci-Fi Channel before it became Sy-Fy, Halmark, and the Jim Henson company, the show was about human astronaut John Crighton, who, due to a weird accident, got blasted through a wormhole to the other side of the universe. He was subsequently picked up by a living ship that had been acting as a prison vessel and was forced to get along with the various aliens onboard who were escaping their own prison sentences.
Sci-Fi canceled Farscape just before the fourth season started airing. Which was a little odd since it was, at the time, one of the network’s highest rated shows, but it was also one of its most expensive shows. Various backers came up with funding for a three hour mini-series to wrap up the story (which is a good thing, because without The Peacekeeper Wars, the whole thing ends on a real downer of a cliffhanger).