Hemp is not legal because there is no way in hell that the government can tax it.

Any nitwit with a vacant bit of the worst most marginal land in every climate, under the worst possible conditions can grow hemp.

Hence controlling it, as we do alcohol, is damned near impossible.

So...the only way for the governments to make any money on this drug is to keep it illegal so they can periodically harvest money from stoners and the businessmen who serve that market.