Individual flights all have seats priced at various levels. When the lowest priced tickets are gone for a specific flight, the fare jumps to the next higher level. While sales are possible, airlines have less reason to do that on popular routes where planes tend to already be fully booked. The best prices, non-stops and reasonable departure times are likely the first to sell. I have no interest in having to get up at 3am on our day of departure to race off to the airport for an early flight.