This is only a mere personal view on the matter you presented.
It's because rural municipalities cannot afford to employ said health care professionals. Employing someone to do something for you requires a corresponding amount: salary.
Another possible reason could be the lack of facilities in the rural areas. If there are no facilities and hospitals, where would the health professionals do their work? We have nurses, medical technicians, midwives and physicians/doctors among others. They can do field work, yes. But a clinic is still one of the basic needs.
By the way, I think every municipality is entitled to have a rural health unit (RHU) which monitors the health of the community, be it urban or rural. They do not employ specialists but there are still the general practitioner doctor, nurses, and midwives to cater to the needs of the community.