Generally when trading isn't offering to US retail, but available to Institutional it's about reciprocal compliance agreements. Korean futures are CFTC approved for US retail, even though they just got tagged for by the CFTC for regulatory shortcomings. Last I looked no reciprocal agreement with the SEC. Also don't confuse market cap. and volume with liquidity. Kospi blows almost everything out by volume, but it really isn't very liquid and I suspect that is part of why no regulator has look to the reciprocal agreement completed.
In the '80s one of the most actively traded names in the US was TMX(Telefonos Mexixo) and Mexico had fairly tight currency restrictions. Most days it was the most Stock and options traded in the US and almost all the volume was an arb on the currency restrictions. Large holders and small investors alike were doing conversions to arb the restrictions. There is more granularity to the strategy, but I suspect there is little appetite from either regulator to make it trade able here._