Esperanto only has a thousand or so native speakers, but two million people can speak it. I speak Esperanto, too, but it's far from being as good as Latin. It's a mix of the languages of Europe, Dr Zamenhof took the largest languages of the most wide-spread language families (Romance: Italian, French, Spanish; Germanic: German, English; Slavic: Russian) and mixed them together to something as easy to learn and master as possible.There's Esperanto as well.
However, it did not attract mainstream attention. Two million "native" speakers is far from mainstream.
There are other constructed languages designed for international communication and to be easy to learn, like Volapük, Interlingua, Communicationssprache, Wede and Lidepla. All of them didn't have any success. Most of them are from the time around 1900 AD, when French as language of diplomacy had already declined while English hadn't taken off yet.
Volapük was based on European languages too, especially English but also much of French, German and Latin. It was to hard to learn because most words consisted of just one short syllable so they basically sounded the same, which made them hard to remember. And they were changed beyond recognation. "Volapük" means "worldspeech", vol is derived from English "world" and pük from "speak"... yes, I'm serious. Also, the inventor, Dr Schleyer, was very possesive and saw Volapük as his property where he was the boss.
Interlingua is a mix of the Romance languages and sounds like a poor man's version of Latin. No need for that, or? It's creator was once was an Esperantist and even gave Dr Zamenhof advice. But the evolution of Esperanto didn't go the way he wanted so he just started his own project.
Communicationsprache was a butchered-up version of French, because it's from a time when French was the lingua franca of Europe. So... I don't get it why somebody thought it was a good idea to create a new version of French instead of continuing to use regular French, I mean, where's the difference? And why was the name "Communicationssprache" in German (communication language)?
Wede was created by Imperial Germany because they thought they were going to win World War I and become the foremost global superpower. Of all conlangs it was the least creative one because it's inventor just took Standard German and dumbed it down a bit. Why? Did he think people would be too dumb to master the original?
Lidepla ("lingwa de planeta", which means "language of the planet") is being worked on at a Russian university since 2006 and is based on the dozen most widely spoken languages on Earth. Basically, it's Esperanto 2.0. There are short-stories, poems and songs in this language and a few dozen people speak it.