Because the British had guaranteed the rights of the French Canadians to practice Catholicism and they also had recently increased the territory available to the Canadians for expansion. The Americans could not provide similar guanantees due to natural skepticism of allying with the puritanical New Englanders and of course there was no guarantee the quest for independence would be successful, and even if it was, the borders of the newly independent nation would be anything but certain at the start of the indenepdence movement.
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