Matt Groening on Futurama, Simpsons and Fox
I think what really happened is that Fox felt it needed to follow through on its commitment to Family Guy, which had been promised the slot after the Simpsons as part of the signing deal. Remember, Futurama was brought in after Family Guy flopped the first time. I don't know whether they did this because of contractual reasons, or because they really believed in the show, but it necessitated moving Futurama out of the post-Simpsons timeslot. This was the genesis of the unprecedented 7-8 Pacific/6-7 Central Sunday timeslot, and they moved in King of the Hill to shore up the lead-in.
Unfortunately, after Family Guy died on the vine the second time, they replaced it with awful Malcolm in the Middle instead of going back to Futurama, by then having built something of an audience at the earlier timeslot (or wanting to use the "incubator" slot for another show).