Because governments don't like people having fun and have an Puritanical view on all recreational drugs besides alcohol, tobacco, and coffee. Also because they, and I guess society as a whole, see a(n arbitrary) distinction between (recreational) "drugs" for the drug-user as bad, but "medicines" for the patient as good. Read Thomas Szasz's "Ceremonial Chemistry" desu.
Some countries have attempted to do just this in Europe and in New Zealand recently with a guy named Matt Bowden. More liberal countries like the Netherlands have always had a softer stance on at least what they consider "soft drugs".
It's a similar thing for developing drugs to improve cognitive function as well, although "nootropics" are a little more popular/accepted at the moment than drugs specifically designed to induce euphoria
Another reason, since you mention "happiness", may be the pharmaceutical companies and the prevalence of SSRI drugs and other antidepressants. Those are seen as the "acceptable" answer to this solution, rather than potentially better/safer things for some people