because it wasn't visible with the unaided eye.
Huge Asteroid 2005 YU55 Zips by Earth in Rare Close Flyby
In astronomical terms, 324,600 kilometers is a near miss...
... of the Used, Abused, and Utterly Screwed Up.
A Secular Franciscan looks at the world... with a more jaundiced eye than ever... and lots of ellipses for you to fill in the missing text... (with thanks to Thomas S. Klise for the title)