As if U.S. domestic drug policy wasn't screwed up enough, there are the geopolitics.
As Ryan Grim notes in This Is Your Country on Drugs, eradicating any foreign drug crop seen as being inimicable to American interests is nearly impossible. This is because it is obvious to any farmer -- whether a Bolivian growing coca or an Afghan growing opium poppies -- that if it were not for American demand for his harvest, he'd have no reason to be in the business.
The collision between foreign policy and drug policy is nowhere more evident than what happened in Mexico and Afghanistan in the mid-1970s.
The DEA-directed spraying of huge Mexican poppy fields reduced the flow of opium and heroin into the U.S., but the insatiable American demand was met by Afghanistan and Southeast Asia. While the DEA was tamping down the Mexican supply the CIA was encouraging cultivation in Afghanistan in the service of supplying anti-Communist mujahideen warriors with cash to buy weapons and other materiel for their insurgency against the Soviet occupation.
Today eradication is again the favored policy in Afghanistan and the occupier is the U.S.