Well, I mean, really.
It's not just about the longevity, though that's not to be belittled. It's not about the animation, though it's way better than most people give The Simpsons credit for. It's not about that amazing theme song, which we've all heard so many times that we forget just how bonkers it is. It's about the writing. It's about the characters. In its prime from season 3 to season 8, The Simpsons was not only the best sitcom on television, it was the best show, period.
What is it that makes The Simpsons so exemplary? It is the summation of every sitcom to come before it, and some that came during it. From The Dick Van Dyke Show to Married... With Children, it can all be found within the world of Springfield. Subsequently, the range of the humour is unmatched. The show easily coasts between traditional jokes, puns, sight gags, slapstick, surrealism, popular references, obscure references... No source of humour has been left unexplored. The balance of low-brow and high-brow is beyond admirable, it's almost impossible. That's why The Simpsons has appealed to so many people for so long; it manages to be stupid and smart, often in the same sentence.
But it isn't just the humour. Futurama, during its initial four-season run, was funnier than The Simpsons has ever been. What Futurama lacks that The Simpsons has in spades is heart. There is a real emotional core at the center of the madness. There are real relationships between the characters, and you grow to care about the characters. For all the differences between them, Bart and Lisa Simpson clearly love one another. We never question why Marge is with Homer; for all the woe he brings upon the family, they do love one another, and they both love their children. That's what matters the most in a traditional family sitcom, that we care for the characters and that we can sense that they care for one another.