When you reduce the GST, you reduce the cost of a consumer product, whether that product comes from Canada or abroad. When you reduce income tax, you reduce the cost of labour. When you reduce income tax, you give people an incentive to work in Canada rather than abroad. Reducing income tax has a greater effect on Canadian economic growth, whereas reducing consumption taxes has an effect on Canadian economic growth but also on production abroad.
On April 7th, 2008. See this statement in context.