Why are some people immune to RSV and others are not?

Immunity is partial. Rsv is a crafty virus that settles into the outer layers of airways like warts on your skin. It doesn't dive into the blood & stimulate your immune system head on, so you gradually get better at fending it off after repeated infections. It is possible for you to get it almost every season, with less invasion of your nose to the point you may not be aware you carry it.(but you may still spread it.).
Previous infection. Generally, infection with RSV (respiratory syncytial virus, the cause of bronchiolitis) leads to immunity to that strain (there are two) of the virus. The virus usually infects nearly all children sometime in their first two winters. The symptoms are worse for children who are smaller, were premature, or have underlying heart or lung disease. In adults, RSV infection looks like a common cold.