Not sure if that is the reason why. On Saturday and Sunday, there were no slots available for 12/21-12/24. Today there schedule was wide open every day from 12 pm to 7 pm (its hours) through 12/24. Plus there is no penalty in cancelling an appointment nor does one have to pay upfront to even reserve an appointment. All you need is a name, birthdate and phone number to book an appointment.

Stopped by today since I was in the area and they said they are not concerned with the actual time booked, but just show up within 72 hours before flight.

Just checked the schedule again, and pretty much the only availability for the week is now 12/22 and only 2 time spots on 12/23