If you're referring to the
city of Saint John, rather than the Saint John CMA, the big difference between SJ and it's two NB cousins is that Saint John is losing population to it's more prosperous suburbs of Rothesay and Quispamsis. The Saint John CMA itself isn't growing, but the population is relatively stable.
The problem of SJ therefore is that it resembles a lot of rust belt cities in the northern US. I do not mean this in any purjorative sense whatsoever, but it is a simple truth that Saint John has trouble competing with it's suburbs for population growth. Saint John is directly on the Bay of Fundy, is frequently cold and foggy in the summertime, is dominated by heavy industry and has an unreasonably high tax rate (due to the heavy industries in the city not being made to pay their fair share). The suburbs on the other hand are located on the picturesque Kennebecasis River, are far enough from the sea to not have to deal with fog in the summer, have no heavy industry and have more reasonable taxation (spread more equitably, no need to subsidize heavy industry).
Because of this dichotomy, those people who can afford to flee the city for the Kennebecasis Valley do so, leaving behind a rump population of poorer individuals in the city proper. I probably have made the situation gloomier than it sounds, and the uptown area of SJ is currently undergoing an urban renaissance, so there is hope, but SJ's struggles will continue.
Ideally the solution should be to amalgamate the entire CMA as a single city, but this is a political issue that no politician will touch with a 3.2m pole..........