[loufe]

You can do a lot of reading on the subject. One thing you'll find repeated is the FLQ crisis' effect on housing. Around that time a large movement of anglophones resulted in a huge drop in housing demand in Montréal. The city continued building at a reasonable pace but all those old apartment buildings/low-rises/row houses are kept maintained and thus haven't dropped out of the housing stock. The unfilled housing ratio is higher in Montréal as a result. That is simple economics.