Why didn't they just adopt the NEC?
Because they still do goofy things there
Because we are not the USA
That said we could but why do we want to? I agree we both use the same products and for the most part Canadian and US rules are very similar in intent but there are some differences.
A province coould adopt the NEC but I think that could create a lot of legal problems for any installation that met CEC but missed NEC or vice versa.
Training. All those Canadian electricians would need to learn the NEC rules.