Current technology means that whilst many aircraft are capable of landing in even zero visibility, the radio signals needed to effect such a landing will be affected by nearby objects, such as other aircraft.
To prevent this interference aircraft needed greater than normal separation on approach, and the area around the runway must be kept clear. This means that landing rates are greatly reduced, hence delays and cancellations.
With the gradual increase in the use of mls instead of ils this problem should lessen.