Pseudoephedrine restrictions drove the search for new chemistry and the new chemistry brought in the large scale labs.
I suspect higher purity & potency of street drugs has much more to do with more sophisticated operators operating outside of the US than strict prohibition. Same with fentanyl.
I believe this explanation is too simplistic...
"the harder the enforcement, the harder the drugs."
https://pubchem.ncbi.nlm.nih.gov/compound/Pyrovalerone
"Pyrovalerone is a DEA Schedule V controlled substance. Substances in the DEA Schedule V have a low potential for abuse relative to substances listed in Schedule IV and consist primarily of preparations containing limited quantities of certain narcotics."