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It would make a lot of criminals rich.

Illegalising cigarettes in one country doesn't stop them from being perfectly legal in other countries. Even these days, people still smuggle millions of cigarettes into different countries because they know people will buy them. People could still buy them legally, but they'd rather shave a few pounds/dollars/whatever off the price and buy them on the black market.

Taxes would have to be raised on other stuff to make up for all the tax lost by illegalising cigarettes, which would lead to public outrage because people who don't smoke don't like having to pay for iron lungs for people who are still insisting on smoking.