Poetry is written in Esperanto, people make love in Esperanto, and the vocabulary has expanded to many thousands of words. People who claim Esperanto doesn’t function as a normal human language are people who don’t speak Esperanto. I take it you don’t know much about it, since you used the term “designers” (it was designed by a single person.)

But Esperanto doesn’t need to fulfill all the same functions as national languages to succeed. It was never meant to replace national languages, after all. It was only meant to allow people to communicate who otherwise have no language in common.

Ultimately it failed because people

1. didn’t believe it could work (notably elites, who make policy about what to teach children in school, did not endorse it) and/or
2. didn’t like its design (it’s only the second language of its kind ever designed, and its various flaws, both real and imagined, are well-known in the Esperanto community).