It doesn't happen every year.
In short, enumerating means counting. If we really want to count rights-holders, then we need to be able to use other tools that fall under the jurisdiction of the provinces and territories.
Ultimately, this comes down to a terminological choice between the verb “enumerate” and the verb “estimate”. If we are talking about coming up with an estimate, then only the federal government can do that. We would be using a snapshot. If we choose the term “enumerate”, then we really need to go through the provinces to get the exact numbers on an ad hoc basis because they are the ones who have access to the list of school boards, for example.