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.
Interlingua is awesome. See if you can understand this:
Interlingua es un lingua auxiliar international naturalistic basate super le vocabulos commun al major linguas europee e super un grammatica anglo-romance simple, initialmente publicate in 1951 per International Auxiliary Language Association (IALA). Appellate a vices Interlingua de IALA pro distinguer lo del altere usos del parola, illo es le subjecto de iste articulo e le lingua de iste encyclopedia integre.