It's fascinating to read about how Esperanto got adopted so much by people in the "counter-culture", for lack of a better term. What enabled that? It is hard to imagine an artificial language being met with so much enthusiasm today.
In my own experience, people who speak Esperanto are more likely to be against the status quo of nationalism and tend to be against the imperialist nature of their own or other large governments. They tend to be for the free travel of people and against strict immigration laws. Those ideologies square well with political movements of communism, anarchism, libertarianism, etcetera which are usually on the fringes of society.
AFAIU this happened because of where Esperanto was created--among the rapidly shifting cultural and political mores of Central Europe. The emigration of both ideas, particularly communism and anarchism, and people (e.g. educated Jews) helped carry it across Europe. The global spread of communism then carried it around the world.
It's unsurprising that the promise of a single, unifying, constructed, internationalist language would appeal to leftists. Which isn't to say leftists are naive, just that it would be much more odd to see initial interest from nationalists or other conservatives.
