Because of all this discussions is that people set down and decided to have a set of standards and rules to classify what would be considered a powered flight. This would have made a nice debate in the late 19th century -- and it probably did.
So they had a few requirements like: it had to be heavier then air, take off on its own, it had to be public, fly for x time and for y distance, that kind of thing. If you research it, you can probably find out.
By what that article says, while the Wright's brothers first flight didn't met all the standards (like taking off by its own means), some later flights did before 1906. They were never officially observed before that, however.