For historical and other reasons, Quebec withdrew from the Canadian organizations that regulate the profession. This left the other provinces together. That is a good part of the answer to your question.
Are the standards the same? No, they are not. The possibility of using the same exam that is used in Quebec was already studied, but since the two health care systems are very different, a large part of the content would have to be adapted. I am not going to go into detail on that. That said, if Quebec were part of the equation, the market would justify the need, given the large number of francophone nurses.
Quebec has its own health care system. Compared to the system in the rest of Canada, the health care system in Quebec is even more different than the American health care system, on which the NCLEX-RN exam that was imposed on us is based.