This is because the damages caused by such an object depends on many factors and not just on mass. Depending on its collision trajectory, composition and its mass an asteroid of this size could do much more damage even than the meteor that exploded over Russia last year.
This Sunday afternoon the asteroid will pass close to Earth by 22,000 or 25,000 miles. That is one tenth of the distance between the Moon and our planet. This is such a close call that anyone with a telescope could see it briefly if they know when to look for it.
Asteroid 2014 RC was first time spotted last Sunday mainly because of its size which made it harder to be detected. Such asteroids come by Earth once every few hundred years so if you have your telescope nearby be sure to keep an eye on the sky.