Airports benefit from having passengers come in, and so part of use it or lose it is ensuring that at very busy airports slots are as full as possible. Upstarts can and will take slots at such airports because they are worth their weight in gold; LHR to JFK makes $1B in annual profit for British Airways alone, for example.

Airports also benefit from stability, though; airlines taking up a large percentage of traffic at a hub drive more traffic than if all those slots were split due to network effects. And to some degree people who transit airports tend to transit the same ones, so there are downsides to hectic, constant auctions of slots.

The main issue is that COVID has made demand totally dry up across the board, and so the inherent assumption, that giving away a slot will bring in passenger traffic where there is none, has fallen apart.