Why DOCTORS are the best administrators / managers of a HEALTHCARE facility than the managers from BUSINESS SCHOOLS?

It is because they know exactly what they want in a healthcare facility rather than people from business school.

An opinion, but usually physicians are not the best hospital managers and administrators. Not if you want efficiency, analysis about return on investment and hard decisions on cost control. It's an older model. Physicians are trained with a different mission - the practice of medicine. Hospitals must be managed well so that doctors have the resources needed to provide a high level patient care and carry out their mission. There's of course the role of a Chief Medical Officer (CMO) and subordinates (don't ask a business manager to give you a clinical diagnosis), but unless fully subsidized where costs don't matter, it's best to have a cooperative division of labor and people operating in their own fields of expertise.