Because it's the third-largest city in Canada, and there are no megacities in Canada to begin with.
It's not competing for people with Los Angeles or San Francisco, it's competing with other Canadian cities. It's also quite a bit more land-constrained than any other west coast city, so even if there
were a larger pool to draw from it'd eventually run into barriers for easy, low-cost growth (which it already has at 3 million people).