This is because dentists in germany can charge the services rendered quarterly to the state insurance, but they only get paid a flat flee per insured patient by the state insurance. It means doctors effectively cover the diff/take the risk of unhealthy patients until they are covered. So they have to balance with private insurance income.
There is no link between getting appointments easier with private insurance and the quality of the medical advice you are getting. I might be wrong, but it feels like you had bad experiences with some doctors and took it personally.
