There are no specific requirement, at least as far as I understand, but simply an analysis of each situation.
In this case, EK would have the benefit of severing several Cdn cities and syphoning traffic from Cdn (and Alliance) carriers. Canada would have only Dubai or AUH to serve and since the cities are so close they can realistically be considered one destination.
The benefit is lop sided. There are several remedies that can be negotiated to even the benefit but I am not sure the UAE is willing.
In any case, it can be solved but it has to be negotiated. The UAE/EK has been public in trying to get the airport authorities and cities to push the Federal Govt into allowing the flights and I imagine those types of tactics do not go down well.