>>1
It is not hard to learn, precisely because it is a language "optimized" to be as easily learnable as possible (or as less complicated as possible).
In any case, don't believe me, nor anyone, judge by yourself:
http://lernu.net
http://ikurso.net
Why it's not more widespread? Well, historically, good ideas are not quickly adopted, it usually take centuries. Take for example: arabic numerals, metric system, women's suffrage, end of slavery, world peace, etc...