That’s because it wasn’t chosen as a European language. If it were, more people would learn it.

Oh you’re a french businessman looking to expand into Spain? Sorry, we don’t know french, you’ll have to use one of the official languages to do the paperwork, which includes esperanto.

You would need to pay a spanish-speaking lawyer. Then a German one, then an English one. Or you could pay a single esperanto-speaking one that would be accepted in any European country.

This would incentivize lawyers to learn esperanto. You could do similar things for other fields. Eventually (after a LOT of time), it would just make sense to do daily life in esperanto.