Fentanyl is a thing also because it's easier to make than heroin because the reactants needed are easier to get your hands on, therefore cheaper for the dealer's/cartel's/gang's chemists to make.

On the more potent point, almost(?) all of the more potent opioids are derivatives of fentanyl and iirc use fentanyl in the synthesis so probably won't become a street drug. Also they're so potent they really are only good for killing people. (Honestly I'm suprised fentanyl is a thing because the active dose is like tens of micrograms and requires expensive equipment to measure safely. But I doubt drug cartels care if they're customers are poisoned by their product.)