Ludwig Zamenhof created Esperanto in 1887 with the dream that it would be a language easy enough for everyone to learn so that everyone could communicate, whatever their native language might be. That dream never became reality, yet Esperanto is not the dead language many people assume it to be. Thousands around the world do speak Esperanto, and there is even a village in Brazil where it is the primary language.
Esther Schor describes both Esperanto's past and present in Bridge of Words: Esperanto and the Dream of a Universal Language.
At the end of this book she describes Esperanto as a language that is "radically chosen." And that may describe the problem it has faced since 1887. Except in that village in Brazil and in a relative few families around the world, it is not a language people learn as children. When someone chooses to learn a second language, that language is likely to be the one that will prove most useful. Around the world, that is usually English. In English-speaking areas, it is likely to be French, Spanish or German. Esperanto is a radical choice. If you want to learn the language, you may have to seek out someone to speak it with, which explains the appeal of those Esperanto conferences that Schor visits around the world to interview people who speak the language.
Although Zamenhof dreamed of a universal language, there have since been attempts to give it a narrower focus. Schor tells of those who have sought to make Esperanto the language of Jews, the language of blacks, the language of the political left (it is not by accident that Schor's travels take her to such places as Hanoi, Havana and Eastern Europe), the language of peace, the language of the Baha'i faith and the language of spiritism, among others.
She frequently refers to Esperanto as a movement, which may describe another problem it faces. Most of us want a language that allows us to make statements understood by others, not a language that itself makes a statement.