The runway will be closed to further traffic because of the fog and therefore LVP's are in force, with the aircraft having only just vacated the runway its most likely still within the ILS Cat II protected area.
Had it happened with better weather conditions and a safe margin existed for departing aircraft I am sure ATC would have continued airport ops once everything was deemed safe.
Only my best guess but lets give everyone the benefit of doubt, at least its not a major incident/accident only a few delays.