futurama was not canceled because of ratings.
Even by the fourth season Futurama was still being aired erratically. This was parodied in the opening sequence of the last episode of Season 4 with a picture of Fry, Leela, and Bender captioned "See You On Some Other Channel." Due to being regularly pre-empted by sporting events, it became difficult to predict when new episodes would air. This erratic schedule resulted in Fox not airing several episodes that had been produced for seasons three and four, instead holding them over for the fifth season. Fox executives were also not supporters of the show. Although Futurama was never officially cancelled, midway through the production of the fourth season, Fox decided to stop buying episodes of Futurama, letting it go out of production before the Fall 2003 lineup.
fox just didn't care about it. the shows ratings were over 10 million for the first two seasons, but still managed six million when it was officially canceled....