Tell that to the IRS, which insists that even if you're working in a foreign country, for a foreign company, on foreign projects that they still deserve their cut.
The IRS has no discretion in this matter; they just deliver the bill.
That's the key part. I don't know the exact legalese, but you probably still qualified as a Canadian resident.
Once you're no longer regarded as a Canadian resident, you no longer owe Revenue Canada anything. It's a clean break.
In the US, no matter how long you live outside the country, as a citizen you still need to file a return and pay taxes on foreign income.