Because no interesting director would dabble with a franchise that has been tarnished 4 times. They would rather tackle something new/fresh or a franchise with a successful movie or two.
Also, I think marvel is nervous of getting someone with very less experience in movie/tv show making for F4. They have taken chances with other characters, but in this case they are playing safe.
Safe pretty much means that you can rule out anything that will blow your minds. A fun serviceable popcorn movie with some decent vfx and action is the best one can hope.
Frankly, I feel they announced the director too soon for F4. Should have waited till comic con or end of the year to announce someone like Adam Winguard, Jordan Vogt Roberts, Wes Ball, Leigh Whannel, Ilya Naishuller etc. Atleast those people have a flair to their direction.