Kratom represents a new chapter in the failed war on drugs that was begun in this country after the prohibition of alcohol was ended.  The prohibition of alcohol was a failed effort that caused a great deal of crime and violence, and the ongoing war on drugs continues to ruin the lives of countless thousands of people who might otherwise go on to make meaningful contributions to society.  If you think these words are extreme, please read Chasing the Scream by Johann Hari, an excellent and detailed documentation of many facets of the war on drugs. 
If you do not have the inclination to read an actual book, you should at least listen to this podcast, in which Mr. Hari is interviewed, and provides an abbreviated version of some of the things he documented in his book.

Kratom originates in southeast Asia, where it has been used for centuries to ward of fatigue and pain, and to treat conditions such as diarrhea. It came to the attention of western science around 1836, when the plant was identified and named by Dutch botanist, Pieter Korthals.

By all reports, kratom has never been regarded as too much of a problem in its native region.  But on August 3, 1943 Thailand passed the Kratom Act 2486, making the possession or sale of the plant illegal.  Contrary to many accounts, this law was not passed to address any sort of social ills caused by the plant, it was explicitly passed because the government’s tax revenues from the sale of opium were falling, as people turned to kratom as a more affordable alternative.  

Police Major General Pin Amornwisaisoradej, a member of the House of Representatives from Lampang in a special meeting on 7 January 1943: “Taxes for opium are high while kratom is currently not being taxed. With the increase of those taxes, people are starting to use kratom instead and this has had a visible impact on our government’s income.” (1)

(1). Asnangkornchai, S. & Siriwong, A. (eds.) 2005. Kratom Plant in Thai Society: Culture, Behavior, Health, Science, Laws
More recently, Thailand has begun to reverse its course with regard to the plant, and in December 2018 it legalized the medical use of both kratom and cannabis.  Recreational use is still illegal.