NASS is primarily a recruiting tool for USNA. USNA is trying to market itself to people and areas that have been historically under-represented. Who are those folks? People from states that typically send very few students to USNA, such as Montana, Wyoming, NDAK, SDAK, Idaho, etc. Students from inner city magnet schools where kids are regularly going to the Ivies but don't seem to know about the opportunities at USNA. Students from various congressional districts that historically have sent zero or very few students to USNA for all sorts of reasons. Likewise, students on the west coast for whom it's harder to visit USNA on a WE will probably get the nod over an equally-qualified student from MD, VA, PA, etc.
The point with NASS is that it is not solely a meritocracy. Thus, USNA doesn't simply take the very best applicants and offer them slots. Rather, USNA tries to target its acceptances with some of the factors above in mind. For this reason, the fact that you don't get offered NASS may have no bearing on whether you get an appointment to USNA. The above said, students from MD and VA and PA also regularly get accepted to NASS.
Finally, just as for USNA in general, there are way more applicants to NASS than there are slots. Many, many qualified students don't get a slot. If this happens to you, don't despair. Don't think your chances are over. Press on with your application b/c, if you're well qualified, you still have a great chance at an appointment -- maybe a better chance than the person who ended up at NASS instead of you.