The 1971 UN Convention on Psychotropic Substances was designed in response to the increasing concern over the diversifying pattern and expansion of drug use. The use, smuggling and trafficking of psychotropic substances flourished in the 1960’s as they were not subject to international control, The 1961 Single Convention was limited in scope to cannabis, opium and the coca leaf, and therefore could not ban the many newly discovered psychotropics (including barbiturates, hallucinogens and amphetamine-like stimulants). Due to their ‘legal’ status and the increasing demand for ‘new’ highs, these psychotropic substances flourished.
The tone of the 1971 Convention is less harsh than that of the 1961 one. Instead of regarding addiction as a ‘serious evil’ as had been the case in the Single Convention, the Psychotropics Convention softens its language, referring to a “concern [for] the public health and social problems resulting from the abuse of certain psychotropic substances.” 
The 1971 Convention also imposed a “weaker control structure” , because of significant pressure from the US and European pharmaceutical industries.
One report quotes a UN staff member of the Division of Narcotic Drugs at the time as stating that “the most important manufacturing and exporting countries tried everything to restrict the scope of control to the minimum and weaken the control measures in such a way that they should not hinder the free international trade.” 
The distinction between narcotic control within the 1961 Convention and psychotropic control in the 1971 Convention remains open to discussion and occurred largely because of lobbying from the pharmaceutical industry to prevent their products becoming restricted by the strict controls of the Single Convention.