The key issue here is the Build-Up of radioactive materials.

Repeated year-in year-out growth in the same soil causes a build-up of Phosphate fertilizers (which have Polonium in them) which is then aborbed by the Tobacco plant.

So it will (and has) take years for the build up to occur and its effects to become noticeable.

The kicker is that the tobacco companies have known this for years. Life is cheap and there will always be "replacement smokers" available in the 3rd World