Why is the Supply of Labour More Elastic in a Single Province/State then it is in the Country as a Whole? Apparently, the federal government has a greater ability than provincial government to increase tax revenues by raising tax rates because the supply of labour is more elastic in a single province than it is in Canada as a whole.

However, why would supply of labour be more elastic in a single province than in the country as a whole?
Because there is freedom of movement between provinces; and the language barrier is low. When international borders are involved, there is much less freedom of movement, and much greater potential for language barriers.

So it's easier for labour to move between provinces if incentivised to do so by a change in relative taxation rates; than it is to move between countries.