The 115th United States Congress: Senate majority - Republican, House majority - Republican, Presidency - Republican.
Blame the party not in power.
Further, there's only so much an admin can focus on and they had a variety of other issues. My point stands which is that one party is very focused on this issue and worked towards concrete measures against it, while the other worked against them.
Additionnally, if cartels can't smuggle drugs through the border, they'll build fentanyl shops inside the USA.
https://cis.org/Arthur/Yuma-Reveals-How-Migrant-Surge-Has-Fu...
"..they'll build fentanyl shops inside the USA..."
US law-enforcement would love this to happen. Damn easy to get a court order and bust up. Damn easy to setup informants to reveal such locations. Its like comparing peanut impacts versus asteriod impacts. You solve the critical asteroid problem first. The peanuts don't really make any significant difference.