Why has it taken astronomers so long to discover the planet QUAOAR?

Space is huge. The object is dim. Under high magnification, even a small amount of space takes a very long time to scan. Plus the object is moving so it's in a different place. That's one way such objects can be identified (they move in relation to the fixed stars on photographs). Note in the article below it was photographed in the 1950's.

http://en.wikipedia.org/wiki/50000_Quaoar

Thankyou very much lance.