European countries are often similarly diverse and don't have the same prison population. Also, when you are talking about numbers in percents of population the amount of executions to make a real difference in population would be staggering and border on genocide, which, outside of a few exceptions, isn't the case.
Actually, European countries are not nearly as diverse as the USA with the exception of the UK. The best analysis I can find on the subject is [1]. I disagree with the point I think chrshawkes is making but the diversity claim is valid.
