Politics and economics.
20% of the tariff being imposed on China is explicitly being attributed to them being a leading source of illegal drug precursor chemicals, most notably, but not only, fentanyl.
US officials have said after a period of sustained action to stop being a major drug precursor supplier, it will be reduced, then removed.
The drug precursor business, including synthetic opioids and AAS, are a tiny fraction compared to the business lost to higher tariff rates, so right after the last election, they started to crack down on that industry to try and head this off.
It's showing signs of having an impact, based on what we're experiencing, and things like fentanyl purity dropping (it's being cut with heroin lol).
Personally I really doubt there will be an "all clear" from the Chinese authorities allowing production of illegal drug precursers back up. It's bad for business, a violation of the anti-drug treaties they're a part of, and with China looking to dominate international industries like (legit) pharmaceuticals, it's far more important ($$$) to keep trade ties open.