It's for the reasons you've described. For one thing, it is embedded and more difficult to change. For another, in the past, ministerial directives were not always transparent. We know about only the 2009 and 2011 ministerial directives through the use of access to information. As a policy matter, the government is more proactively disclosing ministerial directives, but of course it would be nice to ensure that going forward there was always transparency in this area. Again, then, there's a transparency aspect.
On December 5th, 2017. See this statement in context.