Esperanto was invented by an opthamologist, L. L. Zamenhof, to be a universal second (and maybe eventually first) language that would overcome the "curse of Babel", so many different tongues in use that people cannot communicate. Being an artificial language there would be one codified grammar that everyone would use instead of the many dialectical variations seen in natural languages.
Only Zamenhof, while multi-lingual, was no linguist and did a mediocre job of designing the language. In his (partial) defense he was one of the first to try this (there were a few earlier projects), artificial language design was not trendy the way it seems today.
And so for a universal, common language Esperanto has had a tendency to generate new dialects (Ido, Romániço, etc.) often due the inadequacies of Zamenhof's original specification.
There are a number of significant design flaws that make this "easy to learn" language unnecessarily hard. The transitivity of verbs for example requires memorizing the semi-arbitrary rule assignments for hundreds of verbs, and most Esperanto users make frequent errors. Also the actual interpretation of verbs was not properly defined by Zamenhof, whether they express tenses (past, present, future) or aspects (whether it is completed or on-going). Zamenhof apparently did not understand the distinction himself and wrote contradictory things. In fact his grammar is often vague and numerous controversies have developed over the years.
Then there was the wholly unnecessary inclusion of gender for nouns. Zamenhof apparently did this because the languages he was familiar with did this, but the gender assignments are arbitrary, add nothing of a value to the language, require memorization, and are a problem that must be decided with each newly coined word. As a result the language in use has diverged from the official grammar and dictionary, with the conversion of most "male" gendered words to neutral. And this has led to a dialectical split in the language with people who want to simply eliminate gender (or at least the male gender) and those that want to preserve the original specification (such as it is).