But why haven't smoking been banned as an illegal drug?

This is because of the right people have to make decisions including the one of damaging their own body. Never the less, it is a problem for many governments as passive smoking is attempting at other people health. The cloud of smoke that a smoker exhale is far more dangerous than the one that is directly inhale by smokers. For an infant which his age is below 2 years passive smoking can cause him respiratory infections and even death. Passive smoking on children causes them a high risk of getting asthma, bronchitis or to develop allergies. Passive smoking on adults causes them an increased risk of lung cancer.
Another reason why smoking isn’t banned is because of the money that the government make on tobacco taxes. But tobacco also is a cost for the government as it has to spend a lot of money on the large number of people who have smoking related problems and diseases.

Tobacco companies employ thousands of people in poor countries as India and if smoking is banned all this people would be unemployed as cigarettes wouldn’t be produced any more at least legally.