I just noticed this a few days ago as well.
My assumption is that the main reason is to allow Intl => Intl. and US => Intl. connections without having to go through Canadian immigration.
In the old setup, this was impossible as anybody doing e.g. LGA-YUL-CDG could just walk out landside into Canada once they arrived at their CDG departure gate, which was completely open into the domestic concourse and from there the exit. Consequently, connecting pax needed to pass Canadian immigration in any case.
Edited to add: One of the unfortunate consequences of this setup is that the Priority Pass / Contract lounges in the intl. concourse are now no longer accessible for domestic departing pax.