liquid sunshine,
It's far more straightforward from a scheduling, and pax assistance, point of view to take the hit early and cancel some flights as soon as practicable, rather than hoping we in ATC manage to exceed capacity.
It's far better to cancel early and have the rest of the day to rebook passengers on subsequent flights (maybe put a 321 on a certain sector instead of a 319), rather than find it's approaching 2330 when the LHR night jet ban starts and find that several flights cannot depart and therefore hundreds of passengers need rebooking on to tomorrow's flights and putting up in a hotel, not to mention the fact that now lots of a/c are not spending the night where they should be, so the first rotation the next day is disrupted.