For a country that relies and cares so much about tourism, marijuana opens up a lot more of it, way more than taking a small tax of alcohol. Taking 300 baht off alcohol or allow alcohol sales in the middle of the day is not going to change really anyone's mind about where they travel, but open up a whole new industry in a region of the world where these things can come to the death penalty, and you'll get a whole lot of new tourists.
We have yet to see mass casualties, road conditions go any worse, mass chaos throughout the country. You just need better regulation. It not need be an all or nothing enterprise.
Then again, this is Thailand. When we westerners attach our mindset with those of the Thai people, we get lost in the whole system.