>I wonder why that is?
Around the turn of the 20th Century, Esperanto was incredibly significant in the Internationalist Communist movement.
Lots of Chinese and Japanese Communists knew Esperanto, and many works of Communist literature had been translated into Esperanto but not their languages, so it was very good for spreading theory and communication between revolutionaries of different languages.
Additionally there were works of Communist literature written in Esperanto to begin with out east, not even in their native languages, but it's not as if that was every work.
The Japanese in particular liked Esperanto (and continue to),
they liked it so much so that even this one ex-Communist turned Fascist who trying couping the Japanese Imperial government for not being Fascist enough wanted to make Esperanto the sole language of the Co-Prosperity sphere:
replacing the languages of the imperially dominated in China and Korea, but also replacing even Japanese as he thought Esperanto to be a superior language.
Additionally, the Nazis and European Fascists banned the usage of the language as a tool for its role as a tool of 'International Jewish Bolshevism' or something to that effect I'm sure,
and I'm not sure if that's the reason why the language sort of stopped growing in prevalence, but I'm sure it succeeded in repressing it to some degree.
Anyhow, its role in Communism's history is likely why its still marginally more popular today in states which identify as being Socialist I suppose.