In the Outskirts of the Neighborhood

I have known that astronomers had discovered several large objects in the Kuiper Belt, the region of space just beyond Neptune's orbit. However, a post on the Astronomy Picture of the Day site recently showed the evidence for Orcus, which was discovered in 2004.

Orcus is a small celestial body much like Pluto. They have similar masses and the plane and distance of orbit are very close in magnitude also. They're so similar to the other that Orcus is often called the "anti-Pluto" because of the two's symmetry and the fact that the body orbits away from Pluto.



Orbits of Pluto, Orcus, and Neptune
From Wikipedia:
Orcus in blue
Pluto in red
Neptune in grey (shown as a circle to represent the ecliptic)
Orbits plotted in brighter colours above the ecliptic and darker below.
Major axis drawn showing perihelia (q) and aphelia (Q) and the dates of passage.

Besides the symmetry of the two bodies, what I find interesting is that Pluto was discovered in 1930 and Orcus in 2004. It took 74 years to find Orcus. In terms of absolute brightness, Pluto is roughly 16 times brighter. But, considering all the technological advances and all the "eyes" which are looking to the sky, why didn't we spot Orcus sooner? This makes me think We know and we don't know.

Further links:
Orcus Discovery Frames
Orcus on Wikipedia
Pluto on Wikipedia
Astronomy Picture of the Day