Why isn’t Esperanto the global lingua franca?

As is so often the case here: there are some good answers (Vote #1 Andreu Massana’s answer; Vote #2 or #3 Laurie Chilvers’ answer), there are some bad answers, and this is my answer.

• The initial hope of Zamenhof, and indeed of most people in the auxiliary language movement, was that the global language would be imposed top-down, by a committee of wise people.
• That’s not what happened, and that was never likely to happen. Lingua francas are bottom-up affairs. They are bottom-up affairs, to be sure, that harness an existing structure of power. But usually people don’t learn the empire’s language because the empire told them to. They learn it because it’s in their interest to.
• Esperanto, FWIW, endured as a bottom-up affair itself; and as I was discussing with Clarissa Lohr in the related answer to Could Esperanto seriously become the lingua franca?, we wouldn’t have it any other way. Esperantists are now what Zamenhof had called “Esperanto chauvinists”.
• When a language is adopted bottom up:
• Noone cares how perfect the language design is. People are prepared to jump through all sorts of hoops if it will get them advantage. They put up with English spelling, after all.
• When China overtakes America, it’ll be interesting to see whether Chinese As Lingua Franca will put up with Chinese characters. It may well do.
• Noone cares how rich the culture of the empire is. You think all those kids learning English in Indonesia give a damn about Milton?
• Conversely, all those people who assert how culturally vacuous Esperanto is? I give even less of a damn about you. That’s an argument from ignorance.
• Noone cares how flexible and adaptable the language of the empire is. They’re learning it for purely instrumental reasons.
• Noone cares how fair the power imbalance is: yes, the natives of the empire speak the language better than you ever will, but we redress the power imbalance in our face with the tools we have now, not with the tools of future hope.
• People care about their own culture surviving, and keeping the empire’s lingua a second language; but they don’t care as much as you might like. Languages die all the time, after all, and they usually die through the choices made by their speakers.
• What people do care about is how much access to power and money they can get through the lingua franca. That’s why the native languages of empires tend to do quite well. There is a niche for pidgins (such as the original Lingua Franca), when there isn’t a clear dominant player, or when the language contact is more circumstantial; but that isn’t the world we’re talking about now.