Because it has a very bad efficiency rating. Look for a psu that is at LEAST 80+ (efficiency rating) certified.
Also the Extreme series is an older model that isn't even made anymore (at least not in US), it's been replaced by the GX series.
I doubt it actually puts out a REAL 500w.
I already suggested several brands. Get one that's at least 80+ certified.
80+ bronze certified
80+ silver certified
80+ gold certifed
80+ platinum certified
The higher the rating the more expensive they are.