Letters, because that gives you a larger character set, and more permutations. In the US, plates are provided by 55 (or so) subdivisions (states, territories, DC) and they all have different rules. Where I was a kid, 2 letters identified the county and 4 digits. Where I am now, the pattern was "ZZZ 999" until they used that up, and switched to "001 AAA". That's enough permutations to last over 7 years, the maximum lifetime of a plate. Except for custom plates, the numbers and letters are always segregated.