AUKUS is not limited to the South Pacific though (the scope of the pact actually includes more than just the Pacific) and it is not just about submarines, e.g.,:

Through AUKUS, we have also made significant strides in our trilateral cooperation on advanced capability initiatives: hypersonics and counter-hypersonics, electronic warfare capabilities, cyber, artificial intelligence and autonomy, quantum technologies, and additional undersea capabilities.

Canada IS a Pacific state, has interests in the future of the Indo-Pacific region, and has been very obviously left out of western security plans in the area. Not just AUKUS, but Canada has been left on the sidelines of the Quad as well, e.g.,:

We, the Foreign Ministers of Australia, India and Japan and the Secretary of State of the United States met in Melbourne, Australia on 11 February 2022, for the fourth Quad Foreign Ministers’ Meeting. In meeting, we reaffirm the Quad’s commitment to supporting Indo Pacific countries’ efforts to advance a free and open Indo-Pacific – a region which is inclusive and resilient, and in which states strive to protect the interests of their people, free from coercion.