Same reason any network cancels a good show. It’s all about the bottom line, and if the network isn’t making money on a show, or if they feel they can make MORE money by putting something else in that time slot, that’s what they’re bound to do….the networks are corporations and the sole purpose for the existence of a corporation is to make as much money for the shareholders as possible. Don’t make the mistake of thinking it’s just Fox, look no further than the latest fracas with the Tonight Show where NBC decided to replace Conan O’Brien with former host Jay Leno. O’Brien may not have been everyone’s cup of tea, but his humor was more in touch with the sensibilities of the younger audience, the future of late night entertainment…he was the choice if you wanted to continue to build a franchise and position yourself to be the dominant force in a few years, but if you were worried about today’s bottom line, you know that Leno got better ratings and made more money as the host of the Tonight Show (after having 17 years to build his audience) than Conan did in the 7 months they gave him. They knew Jay was losing them money in his earlier show, they felt they had no choice, they acted they way a corporation is bound to act, preserve the bottom line.
Consider that Fox canceled Family Guy not once, but twice, making it not only the first show to be uncanceled, but the only show to be resurrected two times. And why? Because it wasn’t making Fox enough money in its time slot, but when they put it on DVD, it made them SO much money that they had no choice but to produce new episodes. And ironically, what is on when Futurama would have been on? Well, another show by the creator of Family Guy. The guy they essentially fired twice now programs 3 of 4 of their ½ hour sitcoms every Sunday, some have said his contract could be worth a billion dollars.
Consider what television would look like if network execs programmed what they wanted to program instead of what made money. Fox TV would resemble Fox News…the shows would reflect the values and beliefs (right wing, all about the Benjamins) of the executives and shareholders, and not what sold DVDs and ads on their programming. Because rabid right wingers who want news that reinforces their beliefs (as evidenced by the success of talk radio), Fox was able to carve out a niche in their news where programming could be both popular and in line with their quality standards, but I somehow doubt that these same people who rail against immorality would find shows like Family Guy or Futurama (or even their cash cow, The Simpsons) to be funny. Think of how subversively left wing the Simpsons and Family Guy are, if the people in charge of programming cared about enforcing their idea of quality, these shows wouldn’t see the light of day.
So, Fox is just like the rest of the networks, who are just like all the rest of the corporations when you get right down to it. Last year NBC canceled My Name is Earl, and incredibly subversive, satirical look at trailer trash when one of their own develops a conscience…one of the most consistently funny shows on NBC’s Thursday night line up, which was for years the bigggest deal in all of TV. NBC Thursday night gave us Cheers, Night Court, the Cosby Show, Seinfeld, Friends…and now it gives us The Office and 30 Rock. But those shows are fairly low rated compared to other shows on TV today…society has a taste for “reality” TV rather than scripted comedy, and it’s cheaper to produce, so the sitcom has disappeared in favor of cheaply produced train wrecks that draw on spectacle rather than on insight and creativity. So, shows like My Name is Earl, which had ratings not much worse than 30 Rock and the Office, the closest thing NBC has to hits these days, but they had the opportunity to put on a show with Chevy Chase, who like Jay Leno has been a consistent cash cow for them.
The thing about good quality shows that get canceled is that they generally are very smartly written. Look at another short lived Fox comedy, Get a Life…the show was positively revolutionary…it was offbeat and surreal, and today it’s near legendary, but it lasted 2 seasons. It was too cutting edge for TV. Go back further to another network, ABC which ran a show called Police Squad! for 6 episodes in 1982. The show is a cult classic, and inspired 3 Naked Gun movies which made tons of money and practically remade Leslie Neilsen’s career. But the show never had a chance, you had to think about the humor, unlike the #1 show of 1982, Joanie Loves Chachi.
So to answer the question, why does Fox always cancel the good shows, it’s because quality does not equal popularity, but popularity does equal money, and artists are concerned about quality, corporations are concerned about popularity, and Fox is a coporation. There is really nothing more to it than that whenever ANY network cancels a great show.