The ban on Kratom in the modern era was imposed by its main producer, Thailand. The reasons are not clear at all. On the one hand, the current view of the Transnational Institute (a body related to drug policy) is that it was banned for the avoidance of the loss of income from the substitution of opium by its users. It’s at least true in part and not at most because saying opium in that part of the world has very different cultural connotations.
To begin with, the ill-fated opium war that ended the loss of Hong Kong by China. In that fight, the UK tried to force China to buy opium it bought from other parts of the world, flooding its market. The war ended when, in an intelligent move and after an initial ban with painful consequences, the cultivation and trade of opium in Chinese territory was again allowed, eliminating the need for an outer camel such as the UK, which came to exploit the dangerous combination of two inventions in the Chinese people, the newly created hypodermic syringe and the little previous morphine in the form of salts.
UK marketing in the last period of the opium wars renamed morphine as the opium of Christ in a leap forward when mixing religion, colonization and in this case, addiction. And put the missionaries as their best traders in the area. It was intended to be understood as a breakthrough of cultured peoples injecting morphine against the barbaric pleasure or liking of smoking or eating opium.
The UK jumped into another war. The end of the Second World War was under way in 1943 and one of the lessons learned from it was that the supply of opium was essential for an army. Although morphine has totally artificial synthesis, it is not profitable to produce it in this way just like cocaine and to relieve pain, the standard opioid analgesic must be taken from nature. That is still true today. Germany in its warlike career had attempted to produce new drugs to replace the morphs with the result of methadone (also called dolophine, in a name that comes from Latin and not a tribute to Adolf Hitler as some suggest erroneously) but that resulted to be so potent of action.
Fentanyl did not yet exist, one of the best opioids in terms of its potency by weight and the lack of morphs had its importance in handling the war and its essential supplies. The search for new botanical options from where to take drugs was at its peak.
Opium, which has traditionally been consumed in the area, had a regulated market for decades. The small crop for personal use of opium was common and tolerated with the usual bites as in other crops controlled by local chiefs or authorities.
In that context and in that area of South-East Asia, Kratom was banned by Thailand with a shameful law prohibiting it and requiring that existing trees to be cut.