The short answer is to avoid people signing up only for the month that people need the coverage for. For health insurance, coverage usually begins within a month or two after you enroll in a plan.
This is because doctor visits tend to be more spontaneous than visits to the dentist. You go to the doctor when you are sick. However, because people tend to only visit the dentist about two times a year, it makes it easier to predict when a visit is coming up. A large percentage of dentist visits are for check-ups.
Any follow-up visits tend to fall within a couple weeks of these check-ups. If enrollment was automatic or only took a month, people would be able to predict when their dental visits are and only sign up for dental insurance for those months. The delay ensures that those that receive dental insurance are actually enrolled in a plan for a long period of time.
If you're looking for a comprehensive dental and vision benefits you can get year round, check out Ameritas vision and dental plans.