The FF rights including sub-characters (like Silver Surfer) don't lend themselves to Fox's model of low-budget, low risk films like Logan and Deadpool.
Can you imagine Fox making an R-Rated, low-budget violence and obscenity filled Silver Surfer film?
I was thinking about it some last night and you could pretty much randomly pick any X-Men character:
Cyclops
Gambit
Rogue
Kitty Pryde
Colossus
Sabretooth
Iceman
Beast
Cable
Angel
Juggernaught
Emma Frost
Wolverine
Mystique
Jubilee
Psylocke
The Blob
Storm
X-23
Nightcrawler
etc. etc. etc.
And make a better film that fits Fox's model and Fox's universe and would be received by fans far better than an FF or FF related film.
Fox has all these great characters whose stories haven't even begun to be told yet. They could make three films a year for a hundred years that focus on specific characters and small groupings of characters and they'd never use them all up and they'd never get old.
What can Fox do with the FF that they can't do with X-Men characters? When they've used the FF, they haven't even told their story, so if they want to reinvent characters and tell their own story anyway, why not use the hundreds of characters under the X-Men umbrella that could lead to crossovers etc.?
And the problems they've had making X-Men films haven't been because they don't have enough characters. It's because they aren't using the characters they have properly. They're cramming too many characters into films and not allowing audiences to get to know any of them (except for Wolverine and Deadpool).
I can understand that if they think they can get something out of Marvel, it makes sense to at least try, but when it becomes obvious Marvel doesn't plan to give them anything, they'd have to be fools to try again with the FF when they have so much non-FF material to work with.
A Beast film or a Cyclops film or a Nightcrawler film or an Iceman film or a Collosus film could not only be great on their own, but letting audiences see more of those characters so they can get to know them better would make the team films work better.
The FF is just a distraction from fleshing out the X-Men world properly. They have far too many characters in the X-Verse to give any of them the screen time they deserve. The last thing they need is a whole new set of characters taking screen-time from those X-Verse characters.