The reason scientists didn't see it is because of its size. There are hundreds of thousands of rocks that big (Approx. 10m in diameter) in space, so it would be almost impossible to try and watch each and every one.
The issue is, if it were actually 10m in diameter, the odds are quite big that it would've been burned away before it had hit the ground. Shooting stars are nothing but meteors burning away in the atmosphere, and most of those are actually 10m in diameter. This one must've been bigger than that to reach the earth itself and cause such damage to the surroundings.
Quick quote from the CNN:
"The space agency revised its estimate of the meteor's size upward late Friday from 49 feet (15 meters) to 55 feet (17 meters), and its estimated mass from 7,000 tons to 10,000 tons."
I'll admit, that it's still small, but a bit further in the same news report it says:
"In what astronomers said was an unrelated coincidence, a larger asteroid, called 2012 DA14, passed relatively close to Earth around 2:24 p.m. ET Friday"
Let it be that they could track that asteroid, but they couldn't do that with another asteroid which was probably going to hit the earth? I'll stick to my same point, they could've been able to have at least send out an warning to that area.