I have no inside info, sorry. I can only guess that USNA has basically filled its class. As with all schools, USNA makes offers of appointment knowing that, on average, a certain number will be turned down. I, for example, had three students who declined their appointments this year. If more than expected turn down the appointments, USNA will use the wait list to fill the class. If the number of turndowns is at or less than expected, there isn't likely to be much movement on the wait list.
Given that turndowns were due to USNA on May 1, it stands to reason that the only openings will be for those from the waitlist offered slots who turn them down or people who originally accepted but, for various reasons, can't or don't want to attend at the last minute. Typically, there are very few of these.
These numbers can vary from year to year based on a myriad of circumstances. Thus, one year can see quite a few folks taken from the wait list and the next year there may be zero.
Again, the above reflects only my view. It IS NOT and SHOULD NOT be considered official USNA policy.