Interesting topic right here 
Something to consider is that it’s likely that all psychoactive weed plants were derived from the same single mutation source, I.e. rather than multiple people breeding hemp to be psychoactive, they were all originally derived from the one source. The reason to believe this is that the particular mutation that produces thc, like most point mutation is random with the likelihood of many mutations occurring at the same alleles and producing the same outcomes is exceedingly small i.e.
no amount of selection will help if the genes are not present. Of course the hemp and psychoactive varieties may have interbred and traveled the world that way, but given the general lack of proximity between Europe and Asia I would be more inclined to think that the drug type varieties were carried and propagated for this exact reason rather than starting out as hemp fibre plants. 