I would encourage you to read something like Nicholas Ostler's Empires of the Word. He gives a non-technical account of language dynamics for major languages of the world through history. You can see some of the reasons why some languages prosper, and how and when they decline. This goes some way to understanding the complex answer to the question: what makes a language spread successfully.

To summarise, a language needs prestige and utility in order to continue to attract speakers. One of the interesting phenomenons is the dynamics of conquest. For example, when Alexander the Great swept through the ANE, he left behind vast tracks of Greek-speaking civilisation. However, other conquering empires, such as the Mongols, left relatively few traces of their language in conquered territories.

In some respects the question of language dominance is similar to the issue of language acquisition - one of the main reasons people fail to learn second languages is lack of significant motivation. The most common successful motivators are desires to prosper, identify with prestige groups in society, and pure necessity.

So to go back to Esperanto, yes it was in theory designed to be easy to adopt. But easy for whom? Esperanto's lexicon is dominated by Romance languages, its phonology by Slavic sounds, and its whole milieu is focused on Indo-European languages. It is relatively easy, then, for Europeans to adopt, and its culture and ideology is born out of Eurocentric identity and aspiration. Esperanto adoption is heavily clustered in Europe.

Consider why more people haven't adopted Esperanto, or any other 'universal' language. Firstly, language, culture, and group-identity are closely bound phenomenon. To abandon your language and wholesale-ly embrace Esperanto as your dominant tongue is to re-invent your identity and make drastic changes to your cultural persona. Secondly, the prestige of Esperanto is low, and the economic benefit quite limited, unless you have personal investment in gaining benefit from the Esperanto community. That needs to outweigh the loss of neglecting your native language. Thirdly, the Esperanto communities own desire to be 'culturally-neutral' actually works against it, because it discourages the creation of a genuine Esperanto culture (which I would argue is inevitable), and so the cultural attraction of Esperanto over against other language-cultures is low.

tl, dr: People need very strong persuasive factors to change dominant language significantly, Esperanto and other universal language proposals lack prestige and attraction (as well as a military, economic power, and religious zeal).