Canada and the US are very similar countries in a lot of ways. Wage rates and societal norms are very close in both countries. They are a natural for free trade where each country can rely on comparative advantage for the benefit of both.
Mexico is a different kettle of fish. It's greatest "resource" is cheap labor. When North American companies locate there to take advantage of this it dislocates the labor market in the US and Canada. I do not understand the advantage of an FTA with a failing, low wage, third world country.
To have your question answered, just look at part of the text you wrote: low wage. That advantage falls to the corporations, not the nations or their citizens.