Many companies that provide dental insurance to employees have recently looked for ways to minimize the cost of providing some level of care.  The PPO plans are cheaper plans to the employer, as the amount paid out to dentists is less.

To explain this in other terms, consider this:

    Let's say that you and I work together, and I represent management. You now work  until 5 with an hour off for lunch, or 8 hours a day. You work 5 days a week, and make $10 per hour, or $400 per week.

On Friday before you leave for home, I ask "How would you like to make some more money next week?"

You respond, "How do I do that?"

I answer, "I would like you to come in at 6, and stay until 7. (12 hours of work per day). I want you to work the same on Saturday, and then on Sunday, come in from 8 until 5. You will get 80 hours of work in. And instead of paying you the $10 per hour that you are making now, I want to pay you $6 per hour for the whole 80 hours. You will make $480 for the week, which is more money. What do you say?"

Most of the people that I ask this to decline to work more for less pay. 

This is exactly what a PPO insurance plan is.  The dentists that accept the PPO insurance benefits have to work much more for less pay per patient.  Again, I do not see how dentists can make this plan work and pay their bills, without changing the level of care that they provide.  We do not know how to make the PPO Plans work for the benefit of both the patient and the dentists, so we decline to participate in them.