Fentanyl and its analogues have been discovered to be an attractive cutting agent, since their increased sedative potency can be perceived as strengthening a batch of heroin. Due to prohibition, there is more incentive for producers and distributors to minimize costs and maximize profits, rather than ensure a safe product.
Sellers in the illicit market are constantly weighing costs and profit against potency. When cutting agents like fentanyl and other synthetic opioids are added, this increases the quantity that can be sold. As long as a particular batch produces noticeable effects, demand will remain.