Many American treaties (with other nations) prohibit both/either parties from decriminalizing marijuana among other drugs.

Links:

discusses some of the treaties:

https://www2.nycbar.org/pdf/InternationalDrugControlTreaties...

History of illegalization of pot:

https://www.congress.gov/crs-product/R44782

It's not about decriminalizing marijuana - it's about the absurd assertion that the federal government can regulate what you do personally because "yadda yadda yadda…interstate commerce!"