Marvel has spoken on this.
http://brevoortformspring.tumblr.co...ng-for-the-ff-movie-to-tank-so-fox-will#notes
Some of his earlier posts tonight state that he feels that the Fantastic Four aren't part of the zeitgeist, sales aren't where they should be, particularly with digital copies and collected editions and the property needs a break until it can be popular again. This man is the senior VP of publishing for Marvel.
Fact is, Marvel doesn't want the FF back that much since they don't find it to resonate with audiences right now as much as it used to. Right now Cosmic Marvel is focusing on Guardians and Inhumans which are closer to something like Star Wars, Battlestar Galactica and Star Trek where it's a full space opera.
Fantastic Four is a much harder sell but it might sell well considering that Gravity and Interstellar were successful. The problem is that the FF are Gravity and Intersteller crossed with Flash Gordon, Dune and 2001 and right now, that kind of sci-fi isn't what's popular. FF is too grounded for the space opera crowd and too cosmic for the fans of traditional superheroes.
And any hope that Gravity and Interstellar would make a series about Earth-based sci-fi cool again may have just been crushed with Ted Cruz announced to be the head of the budget committee for NASA and given that he wants to abolish the IRS entirely, it's likely that he'll either work to shut down the space program or at least run it into the ground out of spite. The means that people probably won't want to bother with a movie about astronauts with superpowers.
Fox isn't making this because they think it'll sell. They're making it because their contract was almost up. The only hope that Fox has for this is the international box office since China, Russia and the EU countries are still interested in space.