Well, if you want to get into the why, I'll warn you upfront, it's going to sound like a bunch of conspiracy theories.
I'll deal with USA since that's where I'm from, and many other countries look to us to take the lead on these types of things. Tobacco is big money, but the tobacco companies are only a fraction of that big money. In California, a pack of cigarettes is roughly $5. About $1 goes to the federal excise tax on cigarettes, and $1 goes to the state tax. So for every $5 pack sold, the federal government gets $1 and the state gets $1. On top of that are what are called Master Settlement Agreements, money the tobacco companies agreed to pay, based on cigarette sales, in exchange for an end to lawsuits from individual smokers. In 2015 California received $721,702,269.58, from 1999-2015 California received $13,376,650,346.70. Thats 13 BILLION dollars. Now, add to that, some states like California decided that they wanted money faster so they sold bonds on future MSA payments. What that means is, if the MSA payments aren't as high in the future as they were projected to be, the state of California will owe more money to the investors than they will receive. See the problem there?
I don't want to overload you, so I'll just pose a quick question about the other parties involved in all of this. Can you think of any reason why groups, such as the Campaign for Tobacco Free Kids, would actually not want to eradicate smoking?