How can someone like my dad chew 2 tins for 40 years and not get cancer and some people get irvin like 2 years. His mouth was really bad though!

Good question. From 16 yr old , you are smart , every one who chew , or smoke do not get cancers some times many years of assault on our body cells is needed , which causes damage to dna , immediate repair mechanisms go into action , preventing cancer cell formation . Your dad had good genes, now it is your mission to stop and tell him that you love him and want him to be around for many years , he will change.
Genetics. We respond differently to the same stimuli, including cancer causing agents. Only a minority of the tobacco users get cancer and the difference between users who get cancer and those who do not is mostly due to differences in the genetic make up of people.