It's an equity issue. This stems from a basic problem in Canada's income tax formula. That has been debated at least since the Carter commission in 1966, which recommended that income tax be based on family income. Canada's personal income tax system is based on taxation of the individual rather than the couple, family, or household. We also have a progressive tax rate system, where the greater an individual's income, the higher rate of tax he pays. Those two factors combined cause this very great difference in the income tax paid by very unequal-income couples and equal-income couples.
This basically discriminates against the traditional family lifestyle of the breadwinner and homemaker.