Because this is a major cause of overdoses. When the potency is that high, and you have sketchy clandestine labs and criminal gangs cutting other drugs with it,

Because this is a major cause of overdoses. When the potency is that high, and you have sketchy clandestine labs and criminal gangs cutting other drugs with it, the amount of fentanyl becomes a (very dangerous) unknown. People cutting fentanyl into other drugs are not known for a doing a laboratory job of mixing it.