The guaranty fee is principally in place to provide a level playing field between the private sector insurers and the Canada Mortgage and Housing Corporation, which is also active in the area and insures about a half of the market. Of course, CMHC has an implicit 100% Government of Canada guaranty. So the 90% guaranty to the private insurers was to provide a level playing field between private sector entities and the government's Crown corporation, CMHC.
On October 31st, 2018. See this statement in context.