The average height of man 100 years ago was 5'5", today the average height is 5'8". Why are we getting taller?

We are getting taller because of improved nutrition and health.

Explanation:

Height, as a polygenic trait, is controlled by more than 2 genes in humans. Height as a trait in humans can be determined by the genetic code of an individual as well as the environmental conditions where such individual lives in. Environmental factors such as good and conducive environment, improved standard of living, and most especially nutrition and health.

The most reasonable cause why we seem quite taller than humans of 100 years ago is simply as a result of improved nutrition and health which was made possible by our advances in science and technology. Good standard of living and improved nutrition is responsible for we getting taller.