FANTASTIC FOUR has had multiple books at different points, whether in the 70s when the Thing headlined MARVEL TWO-IN-ONE or later in the 80s when he had his own series, or in the 90s when there were UNLIMITED s and UNPLUGGED, and even in the 200s when we had MARVEL KNIGHTS 4 running alongside the main book.
I think the issue is that, at the moment the zeitgeist has moved past FANTASTIC FOUR for the moment to other characters and series. There’s nostalgia for the concept, and an appreciation for it as a noteworthy series, but none of that helps to move copies on Wednesday necessarily.
But these things often go in cycles. AVENGERS wasn’t popular, then it was, then it wasn’t, then it was. And so on. So if the elements come together properly, FANTASTIC FOUR will eventually have its day in the sun again.