Who knows!
So many factors have to be in place for a cell to mutate into a cancerous one. I guess some of it must be chance.
For example, they think the cancer i had is related to the EBV virus. The cells in the back of the Nasopharynx release an immortal protein because of the virus - but that doesn't makes them cancerous. Some other contributing factor then makes them mutate, and becasue they can't die the mutated cells multiply into a tumour.

Perhaps some day scientists will understand the whys and wherefores. Until then i guess some of us are just unlucky.
& in the view of smoking - everyone knows its not good for your health, cancer, heart disease, lung disease etc. But people still start smoking every day!
Maybe if the reasons some people get off scot free (though with lighter pockets) are discovered then there would be no life style related cancers! & if that reason were ever discovered then cigarettes would be made illegal & where would our government get all there taxes from? *lol*
Now thats just opening another Pandoras box!