The answer is that it's not standard practice and indeed it could introduce some element of uncertainty to people not familiar with the Rio principles. That said, the basic legal test for referring to external documents is that they need to be clear and they need to be available in Canada's official languages. Of course, that's the case for the Rio Declaration.
To your more general point, it's not necessary, given that the way the principle is codified in CEPA is identical, word for word, to the Rio Declaration.