Esperanto is not a particularly easily learnable language to most of the world. It's a very parochial language made by someone whose exposure to language was all European and very strongly focused on specifically East European languages both phonetically and grammatically. English, to take a horrifically terrible language at random, is not much harder to learn for, say, a Chinese speaker than Esperanto would be, but it would be a million times more useful given the rather pathetically small number of Esperanto speakers out there.

If you're going to use a constructed IAL (as opposed to de facto lingua francas like have been historically the case), make one that isn't filled with idiotic things like declension by case, by gender, by number, by tense, by ... Or you're going to have most people in the world ignoring it. Like you already have for Esperanto.