Like someone mentioned before, why did the Fairy type rumors come after Sylveon's reveal? Why weren't there a new type or even new type Eevee rumors beforehand?
Well the Light/Fairy/Sound/Whatever rumors have been floating around forever, not just this generation. When Sylveon popped up it didn't really have much of a stereotypical appearance to the types we currently have, so people brought the rumors back up for the new typing. The evidence just got much better with Smash and the type testing.