No, not everyone becomes an official candidate for admission.
If a preliminary application is opened (usually by applying to NASS also known as Summer Seminar) and is deemed not to be reasonably competitive, that applicant will not receive the "official candidate" notification.
From the USNA admissions website: "If the information provided in your Preliminary Application indicates your record is strong enough, you will become an official candidate for admission and you will receive a candidate information letter as early as the July prior to your high school senior year. Upon completion of your candidate file, the Admissions Board will review your record."
If your scores are too low or something is missing to become an official candidate usually a BGO will reach out to you (or you can to them) to discuss how you can improve your application to become an official candidate.