Why do all US non-military aircraft have the letter N before their registration numbers?

Letter N Before Aircraft Registration

The aircraft registration is made up of a prefix assigned by the International Civil Aviation Organization (ICAO) followed by 1-5 characters. The United States has been assigned the letter N.

Due to the large number of aircraft registered in the United States, the U.S. assigns alpha-numeric identifiers according to the following rules.

N-numbers may only consist of 1 to 5 characters and must start with a number other than zero and can not end in more than two letters. In addition, N-numbers may not contain the letters I or O, due to their close similarity with the numbers 1 and 0.

Each alphabetic character in the suffix can have one of 24 discrete values, while each numeric digit can be one of 10, except the first, which can take on only nine values. This yields a total of 915,399 possible registration numbers in the namespace, though certain combinations are reserved either for government use or for other special purposes.