Why is Easter Island so mysterious?

Easter Island is a mysterious place because slave traders committed a very thorough genocide over the short span of 2 years in the 1860s. When catholic missionaries came in relief efforts it was too late to save them. When asked about what their writings (called "rongorongo", basically hieroglyphs carved into wood) meant, the natives were wary of visitors and were unresponsive. Still, they were expected to convert from paganism to catholic, put on some clothes and burn their rongorongo which probably felt like a threat to the missionaries. With the writings... and this is just my take on it (there are many)... dies a lot of the recorded history of the land. I'd say that adds quite a bit of mystery.

Even before the genocide, though, there were a lot of unanswered questions about when people began settling there, where they came from, what the geologic conditions were (some theories involve the sea level being much much lower, thereby making Easter Island the tallest peak on a very large island).