Problem: fentanyl is Super Potent and you are working with microgram amounts. Too much and your clients stop breathing. Lets just say the quality control laboratories of the distributors are not so good and Viola you have overdoses.
IF your priority is to prevent overdoses then place some comeptitive pressure on the market by supplying quality-controlled (government dope) stuff on the street. It sends the local "entrepreneurs" crazy - but they now must UP their quality control measures accordingly (to have a comeptitive product). Fewer overdoses.
But its a messy business and the idea of government-supplied dope makes most tax paying workers' stomach churn with resentment.
Marijuana, that is sold on the street, is marinated in Fentanyl to increase dependency and the high.