Apparently Marvel Comics decided to stop making FF comics as they don't own the (potentially) lucrative film rights, which they sold off to 20th Century Fox for relatively cheap in the late 90s while undergoing bankruptcy proceedings. Why make new stories in the comics which might fuel the competition's movie coffers? It makes (dollars and) sense, but it still stings.
Stan Lee and Jack Kirby crafted 102 issues together as writer/co-plotter and artist/co-plotter respectively. They gave us not only our flawed quartet of heroes in Reed Richards/Mr. Fantastic, Ben Grimm/The Thing, Susan Storm Richards/Invisible Woman and Johnny Storm/Human Torch, but Doctor Doom, Annihilus and the Negative Zone, the Silver Surfer, Galactus, the Black Panther, Mole Man, the Watcher, Skrulls, the Puppet Master, the Inhumans (Black Bolt, Medusa, Karnak, Gorgon, Triton and Lockjaw), the Frightful Four, and so much more.
There's a gold mine of stories there, enough for several movies. Since Fox tried three times to bring the FF to the big screen and failed, maybe in a few years they might sell the rights back to Marvel so that their film arm Marvel Studios might make a truly great Fantastic Four film.
To the Pogo Plane, everyone!