For many people the twice-a-year trip to visit the dentist is not top on their priority list. Some forget to schedule the appointment and later realize that it's been way longer than six months since they last saw their dentist. By the time they schedule the appointment they are way overdue.
Others schedule the appointment but then cancel as the date approaches. They give many reasons for this last minute cancellation including:
Lack of money
Lack of dental insurance
Illness (their own ...