It's my understanding this is because there is not a single licensing authority in the US, but rather 50 separate ones (one for each state) - each of which would need to get on board with an exchange type deal like there is between the UK and Canada. Administrative/bureaucratic nightmare?
I think it's the fact that Canada has the Commonwealth connection. Canadians resident in the UK can also vote.
But the driving licence thing only goes only so far. A Canadian licence is only converted to automatic cars only unless you have written evidence from the licencing province or territory that you learned on a manual. So if you are Canadian, live in the UK and want full access to manual and automatic, you would be better off to take the test. So unless you only want to drive automatics, it is unlikely to be the convenience for Canadian residents you might imagine.
Secondly, not all provinces or territories accept a straight swap of a UK licence, as it's up to the individual province (10) or territory (3), for the same bureaucratic nightmare Mrs R cites.
And I think Quebec might be one of the 'no' provinces. Last time I checked, BC also didn't accept UK licences to swap. Hope they do, if we ever go back.
