The answer is marketing.  

Marketing expenditures far exceed the amount spent on the development of drugs.  Pharmaceutical representatives bear gifts, incentives and biased information to get physicians to prescribe their products.   

Physicians want to do well for their patients but there is a large menu of drugs available to treat most conditions.  It is difficult for a G.P. to spend the time to ascertain the absolute benefit, absolute risk and numbers needed to treat, which is not readily available, necessary to have an informed discussion with their patient.  

The average physician is seldom  in a true conflict of interest but rather biased towards these heavily promoted products.   

Pharma companies do not spend money to target physicians without a substantial return on the monies so spent.