It is a very long story. But the short version goes like this.
The USAF was at once broken up into three sections in the USA. The East, Mid West, and the West. When the three decided to merge into the East, then to just the USAF.
This probably relates to the International Aikido Federation (IAF) rule - one country, one representative. In the case of the US of A, the USAF is the sole official representative.