How come The Simpsons has a steady decline in quality, yet Futurama dosen't?

I used to be a big Simpsons fan, but around season 10 I think, the humor beacame increasingly dumb, story lines became more "whacky", unnecessary and lazy pop culture references and cameos increased, and so on. I haven't watched it in a couple of years beacuse it pisses me off. Yet Futurama, a show by the same creator has, imho, stayed true to itself and has mantained the quality it had. I know it has run for much less time, but are there any other reasons? Is this very subjective and my idea of the Simpsons might be wrong?