I think part of it is that after the in-pocket Governor General protected the Liberals from a non-confidence vote,. one of these two gets to be defacto Prime Minister without a vote (like Kim Campbell) but on the bad side, will then be annihilated by voters (again, like Kim Campbell) in the upcoming election.
But every time the winner of the Liberal leadership race gets introduced at a lucrative speaking engagement, "Former Prime Minister..." will always precede their name.