Why are mutations that occur in an organisms body's cells not passed on to its offspring?

Mutations do not happen to the sex cells (gamets). When a mutation happens, they effect the organism itself, not the sex cells. If a mutation were to happen to the sex cells, then it would affect the offspring.

Body cells do not go through meiosis. Only sex cells go through meiosis and result in sex cells, egg and sperm. Only sex cells join and carry traits from adult to offspring.