... WHITE AND IN HIS TWENTIES: that is more or less the way they are described in COPYCAT and in many of the law and crime shows that I regularly watch.
Says who?
I'm a white Puerto Rican of 58; are there no serial killers that fit my description, and if the answer is indeed no, why?
God is subtle, but He is not malicious. (Albert Einstein)
Possibly because there are more of them (white guys in their twenties, I mean)? In the US, Canada, and Europe anyway.
More than likely, though, it probably has to do with socioeconomics (privilege and access). White men are among the most privileged people on the planet. Therefore, perhaps they also have the most leisure to indulge in their psychopathic/sociopathic compulsions.
Not having researched the subject much, that's my instinctive guess.
Furthermore, why men over women? That's probably self-evident; men are more aggressive and generally less sensitive.