Why, you ask?

It all comes down to simplicity. The background is white, the numbers/letters are red and the state and slogan are blue. There's nothing fancy about them - no silly pictures of trees (Indiana) or ugly colored backgrounds (Vermont), or fruits (Georgia) or websites (a bunch of states have this) or boring color schemes (Connecticut).

They look good on all cars because they don't take the attention away from the beauty of the car itself (New York).