So the question isn't, "Why does the US have so many more serial killers?" it is "Why does the US categorize so many more people as serial killers?"
That is most certainly not the question. You could deny anything with that kind of logic: the question is not whether the Earth is flat or round, but why so many people categorize it as round.
But if you had taken a look at the Radford numbers, you'd have seen that there is a list which identifies killers according to the same rules, and the pattern is the same. You're just looking for an excuse.
Here are the numbers in a convenient pdf: http://maamodt.asp.radford.edu/Serial Killer Information Center/Serial Killer Statistics.pdf. The base url has more information.
Even if you don't believe the statistics from the other countries, a quick look at the time line dispels the idiotic thought that serial killing is universal.