I don't think it would have been necessary to set up interviews in each of the Maritime provinces. Halifax is an easy enough drive from NB & PEI. Newfoundland pretty much necessitated its own.
Right now I am starting to think Cambridge was used to deflect prying eyes away from Toronto. Relatively speaking, it's close enough to Toronto. Just my opinion. I guess we'll know in the next few days.
Applicants in northern areas would be asked to make their way to the nearest interview. And if there were any in Northern Ontario (for instance, Thunder Bay), they may actually be closer to Winnipeg than Toronto.