Summer
I guess that a lot of them take a time off in order to travel somewhere.
Moreover, there is not a lot of activity going on in MTL during the last two weeks of July.
I don't know how it is actually, but in the past there were more SW (from the States) during summer time. As I don't go downtown very often, I am unable to tell you if it is still the case. For an accurate answer concerning SW during the summer time ask a pro like Oliver.