Why is the Supply of Labour More Elastic in a Single Province/State then it is 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.