Allan:
Because the “natural born” clause is a historic relic of the US revolution which few people in the United States actually understand and which is pointless under the Westminster system of Parliamentary government.
Under the Westminster system, the Prime Minister is the person who has the support of most Members of Parliament. It doesn’t really matter where they were born as long as they do a good job when they are there. If they lose the support of Parliament, they can be kicked out of office with little effort or notice, which keeps Prime Ministers on their best behavior.
Several Prime Ministers of Canada were born in Britain, and one Prime Minister of the UK was born in Canada.
It was just one big, happy British Empire so it was irrelevant where the Prime Minister was born.
The only requirement for a Canadian Prime Minister is that he or she have Canadian citizenship, however obtained, and the UK Prime Minister isn’t even that restricted. A UK Prime Minister can be a citizen of the UK, any Commonwealth country, or Ireland.
David Moe