I can offer three suggestions:
First, serial killers are extremely hard to catch. When an American police force suspects they are dealing with a serial killer, they often find some pretense to haul in the FBI (thank God for the regulation of interstate trade, or whatever that pretense is). Most countries do not have the luxury of calling in an investigative team of the level of competence that the FBI displays. I cite the story of the serial killer in Moscow whose exploits were actually concealed by the KGB because they couldn’t catch him. (Of course, that fellow was a caucasian, too.)
Therefore, I contend, Americans catch more serial killers than most countries. As pointed out above, a plurality of Americans are white. That might get you part of your answer.
Second, the classic FBI profile created by Robert Ressler and John Douglass is that a serial killer is a white male, 25-40, who drives a red vehicle. In other words, they’re looking for white guys from the get-go. You won’t find that for which you do not search.
There is a related factor in this assumption. Most apprehended serial killers are those who target white women. A notable exception was Dahmer, who preyed largely on non-white males, and who was caught practically by accident (one of his victims who temporarily escaped was actually returned to him by the police). One has to ponder the question, “do serial killers prey mostly on white women, or does society search harder for those serial killers who do?” I don’t have an answer for that, so I’ll just say…
…There is the possibility that a form of racism plays a factor. In the 1920’s, a number of people who displayed serial-killer-like behavior found their way into organized crime. Their murderous exploits were largely chalked up to gangland-style killings. We may not be fully cognizant of something similar happening within the inner-city non-white communities.
The third possibility seems to be somewhat exclusive of the first two. That of course is that white males somehow have a predisposition to be serial killers. As we have seen countless times on these boards, it is exceedingly difficult to show any identifiable, uncontestable differences between races when it comes to matters of the brain. It seems to me that you must disprove the first two contentions above before you can go forward with examining the third.