The airlines lose money on the cheapest airfares. They make up for the money-losing airfares with business travelers who buy their tickets at the last minute and pay the extremely high and profitable air fares.

With fewer travelers, the airlines don't want to dig their graves any deeper by continuing to offer the usual low money-losing airfares.

This is all just speculation on my part, but this explanation would explain the lack of cheap airfares in many markets.