Whether Klingon or Esperanto or C, articial languages exert a powerful hold over the human imagination.
Some revolutions are easier to recruit soldiers for than others.
For more than a hundred years, the Esperanto movement has been trying to attract disciples with a vision of a world unencumbered by language barriers - a happier, more peaceful world. The vocabulary is easy to pick up, especially if you speak one of the European languages from which it's drawn. You probably don't know any Esperantists. So because it's small, it's sort of like a group of friends. Estimates of the number of Esperanto speakers worldwide range wildly, from 50,000 to 10 million, but 1 million seems like a reasonable guess - about as many as speak Estonian.