There are all sorts of drugs available and legalising and taxing them could be a significant source of income for the government.
However, if they legalised heroin, most users would end up dead before they had paid enough tax to cover their hospital treatment.
If they legalised cannabis, most users would only use it occasionally.
What's needed is a drug that is addictive, so that people will keep using it frequently despite the inflated price caused by taxation, and causes long term health effects, so that they will hopefully die before they are old enough to claim their state pension.
The only two drugs that fit this description are alcohol and tobacco and, by a remarkable coincidence, they are also the only two drugs that the government has legalised.