This editorial in the Lima (Ohio) News nails it:
Morales’ popularity in Bolivia does have a great deal to do with U.S. drug-control policies, which have disrupted traditional agricultural patterns and customs in South America without reducing the flow of cocaine. The war on drugs undermines civil society in Latin America, promotes violence and corruption, and creates opportunities for ruthless people to make fortunes.
Aside from ending the drug war, there is little the U.S. can or should do to counter the trend.