Because the Laurentian Elite (fucking rebels) have always been more receptive to the idea of the Fenian Raids than most Canadian historians would be willing to admit.
I'm sure that when Biden blew off the BBC with a casual "I'm Irish" then came out as hostile to a trade agreement with the UK Trudeau damn near came.
Essentially speaking, it's for the same reason that he runs in the Papineau riding.
In Canada, as in the States the real political power is reserved almost exclusively to those who have old blood (to the point where the first black US President could trace his ancestry back to a founding puritan colony) and old alliances still exist to this very day.
In this case, it's Lower Canada Rebellion and Fenian which to be frank, is an improvement from the usual Lower Canada Rebellion and Mayflower puritan ancestry although not in a way that's good for Canada as a whole.
Yet they call this the New World.