And I have no way of knowing what future event might bring another language to prominence, or even when. The first reason has to be its ease of acquisition. You can learn Esperanto in about a fifth the time you'd need for most languages. That's because its creator, Dr. Ludwig Zamenhof, understood what kind of hangups people have about learning languages. He wrote just 16 rules to cover spelling and grammar, and they apply without exception.
Few languages come close to Esperanto's simplicity.
Very few people learn it from birth, so hardly anyone can be thought of as having an unfair advantage by virtue of it being their mother tongue. Even they still need another language to talk to people outside their immediate families. Many of those who do learn Esperanto when they're still babies, were born to people who married after intercultural or international romances. Those romances produced living proof of Esperanto's usefulness as a communications tool. If you can fall in love and raise children using Esperanto, the language must work.