They use “troops” because It is a generic term for members of all services. The word “soldier” refers _only_ to Army personnel, and their may be personnel from the other branches there as well. Air Force personnel are only called “airman”, regardless of gender. Navy and Coast Guard personnel are “seaman”, again, regardless of gender. Marines personnel are only referred to as “Marine“. (You _really_ don’t want to get that one wrong.)

A group of personnel from more than one branch of service can be called “servicemen”, service members” or “troops”