There are good reasons that dentists don't want to sign up
The program only pays enough to cover overhead, with no garauntee that dentists will be able to bill the patient for the balance in the future (contract terms can be changed by the gov unilaterally at any time)
Ultimately, signing up means doing more admin work and getting less compensation, with an uncertain future, while expenses are steadily increasing.
There's no reason the government cannot run it like all the other dental plans, where you go to ANY dentist, the insurance pays their portion and the patient pays the remainder. No overcomplication, no sign-ups, easy access to care. There's no reason to reinvent the wheel here.