Why isn't the Fantastic Four more popular? Out of all the comics in the industry it seems that Fantastic Four should be among the most popular. It has several things going for it that only a handful of franchises can match. If you analyze it in depth Fantastic Four has the tools to rival even Batman in popularity.
1. Rich Tradition: The fantastic four series has been written by the best of the best and has memorable runs that should be included amongst the very best in the history of the industry. The initial run by Lee/Kirby started off the series with a bang that is considered by many to be the best in history. Introducing Marvel mainstays such as Dr. Doom, Galactus, Silver Surfer, Skrulls, Red Ghost, Molecule Man, Mole Man, etc...Following Lee we have had runs by Roy Thomas, Marv Wolfman, John Byrne, Walter Simonson, Steve Englehart, Mark Waid, and Carlos Pacheco with recent acclaimed runs by Jonathan Hickman and Matt Fraction.
2. Rogues Gallery: The rogue's gallery of the FF is easily top 5 in all of comics. Not only do you have the most awesome villain ever created; Doctor Doom but you also have a bevy of supernatural villains (Diablo, Mephisto, etc) cosmic foes (Skrulls, Kree, Negative Zone, microverse, etc), seemingly omnipotent beings (Galactus, Abraxas, Celestials, Molecule Man, etc) and mad scientists (Red Ghost, Mad Thinker, etc). In fact; if you simply name your top 10 villains in the Marvel Universe then it's a safe bet that at least one started as a Fantastic Four villain and I'm probably underselling it vastly.
3. Advancement in Comic Science: Between Reed Richards and Doctor Doom the series has advanced the science of comics more than any other book ever. We've had "unstable molecules", microverses, multiverses, omniverses, etc... The "comic science" of the book is extraordinary. The book is most fun when it suspends our disbelief and we wonder what's possible and what's not in comics. The Lee/Kirby run was at the time rivaled only by Challengers of the Unknown in terms of science fiction. Early on the only other place to get these kinds of stories was some of the Sci-Fi serials being published.
4. Family: Fantastic Four is in itself a book full of supporting characters. The book is about a family in which each character can be considered the lead character of the book. We have two characters with the two most popular catchphrases in comics and have each supported their own ongoing series for different time periods. We have the smartest man in comics who could possibly support his own ongoing series if afforded one. There's also the matriarch of the family who is the most powerful member physically of the group. Of the group, Sue Richards is probably least likely to be afforded her own ongoing series, but that doesn't mean she couldn't support several limited series or shouldn't be considered for one. All that and we haven't even mentioned Franklin and Valeria who are the Bart and Lisa of the Marvel Universe and have over the years been fleshed out remarkably into powerful characters on their own and were the pillars of the FF book for a while.
Conclusion: Of every book on the market; the Fantastic Four is perhaps the most underrated franchise in comics. The history and possibilities of the book are so deep and rich that it should be rivaling Batman and X-Men in terms of sales and popularity however for some reason it's relegated to the second tier of the Marvel Universe.