Why does the foreword say this story might be a parable?

The foreward of this story mentions that The Pearl may be a parable because it is largely based on a legend Steinbeck heard in La Paz, Mexico. The legend is based on a poor young man who encounters the psychological and social consequences of sudden fame and fortune, after discovering an extraordinarily valuable pearl........ thus, Steinbeck used the premise of this legend to write the parable the Pearl.