If Esperanto were ever adapted in significant numbers it would develop irregular forms and all the other quirks that make natural languages difficult to learn.
You can read 200 year old texts without problems. Shakespeare is close to five hundred years old and you can still understand it with a little effort.
Think modern Latin: It's not evolving anymore. It's adding words, but its grammar is unchanging.
That's probably a big factor. Maybe Esperanto would achieve similar stability if it were only used by ISO working groups or in other official environments. But if that happens, it ought to come under fire for its Eurocentrism, its sexism, and its general quirkiness, all of which it has been able to skate on because it's never been taken seriously before.