Because he started out as this...
...and turned out to be this...
ECLIPSO started out as both ultra-cheesy ("He's Hero and Villain...All in One!") and a bit uninspired (even writer-creator Bob Haney admitted he cribbed Batman for Eclipso's alter-ego, Bruce Gordon). He was a rather bland bad guy, fiddling about with his black diamond in overly elaborate schemes that only seemed to interest himself. Only a chance confrontation with the Phantom Stranger hinted at something more sinister...
|By the awesome Mike Mignola.|
Turns out, ECLIPSO was God's original Angel of Wrath, and took his job way too far. God punished him by separating his spirit from his power. Eclipso's powers were trapped in a giant black diamond on Apokolips called the Heart of Darkness, while he was banished to the dark side our moon. Somehow, both ended up on Earth, but the black diamond was cut into a thousand identical shards.
|But wait, it gets worse...|
Which is what Eclipso intended. He set out to possess Earth's Mightiest Heroes in "The Darkness Within", and when that plot turned to failure, he corrupted a small South American country, killed a bunch of heroes led by Bruce Gordon, and wrestled the Spectre to a standstill.
Although Eclipso was defeated in the end, no one has taken him for granted since. He has a way of tickling our darkest impulses to the fore, and of all the rebooting of the New52, his reintroduction has been one of the more well-handled ones.
|Yes, this happened.|
In short, Eclipso is the flapping of the butterfly wing that becomes the hurricane, the seductive whisper that fans the flame of want. He is the darkest corner of our soul.
|Let the revels begin, indeed.|