Apparently the language was popular among early 20th century socialist movements because it was of an international character and therefore not associated with any nationality and its use by international socialist organisations wouldn’t show favour to any particular country. It was banned in Nazi Germany and other fascist states because of its association with the left wing, with anti-nationalism, and because its creator was Jewish. It has mostly languished since then but still has around 2 million speakers with about 1,000 native speakers.
I’ll add that the great scattering of Esperantists is in a way a benefit, given the second reason above. If I visit London, where I know no one, I can go online to contact Esperantists who live there, and thus find a local guide. Though we could speak English, we surely would use Esperanto to communicate, and were I to visit (say) Prague, Berlin, Copenhagen, or Helsinki, I could also find local Esperantists as contacts.