Because the US actively opposed the Venezuelan economy and as the Hemisphere's hegemon and virtually the only buyer of it's one export commodity, exercised overwhelming influence. If you choke off the money supply, you kill the economy. https://www.reuters.com/article/us-venezuela-politics-usa-oil-graphic/venezuelan-oil-exports-to-u-s-still-a-primary-source-of-cash-idUSKCN1PJ2CT