Fox was, apparently, trying to capitalize on the popularity of the Simpsons by introducing complimentary cartoons - Futurama was one, King of the Hill is another, and there have been a couple other animated shows they've tried with mixed success. I think King of the Hill is the only one that has actually been successful enough to make it into syndication.

why when Fry went back in time, he saw the Simpsons. Also "The Simpsons" appeared in "King of the Hill" when Bobby's football team played against Bart's team. I thought, like Happy Days (Which spun off "Laverne and Shirley", "Jonnie Loves Cachi", and "Mork and Mindy") they used characters to base it on new shows?

Nah, those were just arbitrary "slice of americana" wink-wink nudge-nudge references. The Simpsons jumped a couple of years ago, I think; they're finding it increasingly hard to come up with whacky and funny storylines. Perhaps when Flanders' wife died. (Death of fairly major characters, especially in a cartoon, can frequently indicate shark jumping in progress.)

Still fun to watch in syndication. I think my favorite episode was where Homer gets a psychotic episode from eating the Ultra Whatever-it-was chili.