We don’t know for sure. The official reason is that it’s due to a new policy on YouTube’s end, regarding royalty-free music.
Supposedly AdRev now has to manually check that the user has a valid license before issuing the claim. This is too much work for them and so are not accepting more “royalty-free” applicant.
However this raises some (unanswered) questions, and it doesn’t seem to fit with some buyers’ experiences. So, we’re kept in the dark.