Why does health insurance stop at the mouth?

Eliot Gelwan, a psychiatrist, reads about problems people in Britain have getting affordable dental care and says he feels their pain:

... None of my MassHealth (the version of Medicaid here) patients have any dental benefits, and it is getting more and more difficult to find even emergency services for them. From time to time, the underlying reason why someone presents to me with a mental health problem such as despondency or suicidality (which MassHealth still pays for) is agonizing dental disease. ...