Why wasn't Esperanto ever globally adopted as the universal language?

Because, believe it or not, people have never needed a universal language.

Learning a language is not easy and most people do not learn another language unless they have a pressing need to use it. For most people, there is no need to learn more than one language (i.e. the language of the place where they live). When there is the need to communicate across different languages, it is more efficient for a few people to learn other languages and translate what is necessary than for an entire society to learn another language.

A universal language would never be agreed upon because some would prefer one choice over the other whether because of ease of acquisition or history or cultural connotations (just using a Latin instead of a Cyrillic script to write in, for instance, can be a political and religious issue even today in some places). Even if it was it would result in the death of other languages since there would be no reason for people to expend the time and energy learning them as well (this is why the spread of English and other colonial/imperial languages has resulted in the death of many other languages).

We already have a universal language -- English. This has been the universal language for at least 100 years. Why would we change to a language that hardly anyone speaks?

The following quote is from the Wikipedia article "English Language":
http://en.wikipedia.org/wiki/English_language
"Because English is so widely spoken, it has been referred to as a "global language." While English is not the official language in many countries, it is the language most often taught as a second language around the world. It is also, by international treaty, the official language for aircraft/airport communication. Its widespread acceptance as a first or second language is the main indication of its global status." (Nov. 12, 2005)

See also this Wikipedia article, "Esperanto as an International Language":
http://en.wikipedia.org/wiki/Esperanto_as_an_international_language
"Esperanto has not lived up to the hopes of its creator, who dreamed of it becoming a universal second language. Many critics say that one's time would be better spent learning English or another natural language that brings significant benefits. Many Esperantists concede that the language has little chance of ever competing with English." (Nov. 12, 2005)
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