Why does one person with these risk factors get shingles while another does not?  

Don't know. Risk factors increase the risk but do not guarantee that you get the condition. Genetic variation and immune system have a lot do with it, and our level of understanding of the process is very limited. There is a vaccine available. Talk to your doctor to see if it is recommended for you.
Immunity. High immunity and prior exposure to help to resist re-infections.