Like other have said, the UK & USA have larger population then all the other countries you listed therefore, the bigger the population, the more serial killers there are.
Also, some police forces in other countries are, compared to our police, poor and may not catch all killers.
I think population is the main reason, if you look at the list for the USA and look how much larger it is compared to the UK you can see what we mean. I can't remember the exact figures of population for the USA but it's larger therefore there's more serial killers.