The reason you would regulate something like this is because airport capacity is a huge bottleneck for air travel where demand always outstrips supply, and ever

The reason you would regulate something like this is because airport capacity is a huge bottleneck for air travel where demand always outstrips supply, and every flight which doesn't run creates a massive opportunity cost for a flight which would have otherwise served a lot of real travelers. If you made it purely a pay-per-slot system, what would prevent some airlines buying a lot more slots than they need, making air travel more expensive and more painful for everyone?