Why is Mexico's wage so low?

Because Mexico is a developing country.

Wages are low because of productivity issues: many jobs and occupations are still labor-intensive, and due to low capitalization (e.g. lack of enough foreign investment), these cannot be improved with automation.

The best example is agriculture. Many people use outmoded methods of farming, such as using the manual plow, instead of modern tractors. They cannot afford to buy a tractor, hence their productivity is low and the amount of money they receive for their harvest is equally low. This is known as the circle of poverty.