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.