I’m a bit puzzled that the recent powwow in Rome was attended by all the American cardinals rather than all the American bishops.

As I misunderstand it, bishop is primarily an office, but cardinal is primarily a personal rank; a cardinal may or may not also be a bishop, in charge of all the churches of a diocese (territory), just as a prince (member of a royal family) may or may not also be a duke (ruler, once upon a time, of a duchy), and vice versa.

How many cardinals are not bishops? How many dioceses have bishops who are not cardinals, and thus were not represented?