Why is it so difficult to discover the earth Trojan? It is discovered only in 2011.

Why is it so late? 

We know where to find it.
2010 TK7 has a diameter of about 300 meters (1,000 ft). Its movements do not bring it any closer to Earth than 20 million kilometers (12.4 million miles), which is more than 50 times the distance to the Moon. (from the wiki entry for it)

That should make it pretty clear? Can you see a fly from 200 km away?