It is being moved to the CIRB now simply because there isn't enough activity at the Canadian Artists and Professional Relations Tribunal to justify maintaining its existence as a separate organization. As you pointed out initially, they both deal with the same subject matter. The understanding is that CIRB will be able to undertake the responsibilities and manage the industrial relations for artists as well as for others in the federal jurisdiction.
On May 28th, 2012. See this statement in context.