There is an element of chance in this. Damage to DNA can occur in many places, some are crucial for the function of the proteins that are encoded and some not. Some location affect production levels of proteins (up or down) and some do not.
If the protein is a tumor suppressor then disabling or lowering it will be bad. If the gene product is an oncogene or grow factor then increasing its amount or changing it (for the worse) it is bad...

Some people are less susceptible to damage and/or can repair or control damage better. That has been well documented and studied.

DNA damage is a fact of life, you cannot escape that no matter what you do. You can however, increase the damage by eternal factors.
In many ways this is like russian roulette, for some, because of genetic predisposition and eternal factors there is a bigger CHANCE that something bad happens, while others get old enough to die from something else.

so why ... is a question you cannot get a full answer for.

M