I wonder if it has anything to do with the length of time you spend on jury duty - in the UK, the average jury duty lasts for 2 weeks, so if your case is closed quickly, then you go back into the pool for another case. However, looking at some of the US jury duty websites, it looks like a lot of people complete their jury service in just one day... which could mean that they call up many more people per year to do it than in the UK.
Just looked around for some stats:
In the UK about 480,000 people are called up per year (about 0.8% of the population), but it looks like in the US, around 500,000 people per state are called up each year, so that's about 25 million people per year (8% of the population) (I can't find any exact statistics for the US though) - so that's 10 times the number of people called up in the US than in the UK.