They only give one per year and there's probably more than one nobel prize worthy achievement every year on average, so they end up with a backlog. Unless some

They only give one per year and there's probably more than one nobel prize worthy achievement every year on average, so they end up with a backlog. Unless something huge happens in a given year like the Higgs, they're probably going to reach back as far as they can for a worthy award to make sure people get the awards before they die.