There was an attempt to make Esperanto the language of Neutral Moresnet.
Most human language is organic in development, and invented languages just don't do that well. People do study it, as DLBarker says, for specific reasons, but bringing together Esperanto speakers takes work and something like an academic or classroom or work environment.
It doesn't spread like natural language, because it isn't a natural language. I've never heard of any cases of someone attempting to raise a child in strong bilingual fashion using Esperanto. In fact, while I know several people who speak it, I don't know anyone who speaks it as their primary language.